Is bedding the AR barrel a myth? What's the best way?

[6.5SH]

Looking on triggershims but cannot find the assortment pack you bought, can you post a link? Thanks

Their site is a bit of a mess but nice people to deal with.

AR-10 AR-15 Shims for Pivot Pin, Bolt Catch, Barrel, Trigger

AR-10 and AR-15 Shims for Pivot or Take-Down Pin, Bolt Catch, Barrel, Suppressor or Compensator Shims, Trigger and Sear Shims, Mil Spec & Large Pin Colt Shims

triggershims.com

ETA:
use the drop down and select the DIY combo then note what sizes you want

 

[Wildfire1550]

You rock! Thanks

 

[Compadre]

When I did mine I had the second recoil lug dovetailed into the barrel after I had bedded the barrel into the forestock. I smoked the barrel down on to the wood then double tapped it from the front of where the second recoil lug was marked to go forward to the muzzle and smoked that down to give me the light float then cut in for the recoil lug and glass bedded the lug with the all faces taped except for the rear face. I always bed on wood and the smoker is the best accessory for doing that.

 

[Tokay444]

When I did mine I had the second recoil lug dovetailed into the barrel after I had bedded the barrel into the forestock. I smoked the barrel down on to the wood then double tapped it from the front of where the second recoil lug was marked to go forward to the muzzle and smoked that down to give me the light float then cut in for the recoil lug and glass bedded the lug with the all faces taped except for the rear face. I always bed on wood and the smoker is the best accessory for doing that.

Uhhh, what?

 

[Rocketvapor]

Where does the 'normal' upper flex?

 

[TheOE800]

Where does the 'normal' upper flex?
View attachment 8312228

There’s an old M4C thread with testing on that subject matter.

 

[rothgyr]

I true em and glue em,... did enough testing in the beginning before and after, there was always improvement, sometimes a farly drastic improvement...I just do it for all..also centering the bolt carrier can be beneficial.

Ok talk to me about centering the bolt carrier. I'm rebuilding my Grendel which is not giving impressive groups, and I'm planning on loctiting it, but I'm curious about how you go about centering a bolt?

 

[45-90]

Ok talk to me about centering the bolt carrier. I'm rebuilding my Grendel which is not giving impressive groups, and I'm planning on loctiting it, but I'm curious about how you go about centering a bolt?

It's centering the bolt carrier, along with a good to top quality barrel, plus all the other procedures one can have a very accurate AR style rifle.
This takes machine tools and precision measuring equipment...not for the average builder.
For those curious, there are videos from AR Armors to help one with understanding the steps, in accurasizing...it's where I learned, buying books, and videos, plus being a machinist with a home machine shop. An advantage most do not have...but one can learn.
If you have access machine tools, and do fixturing, or have a machinist friend show you. It looks something like this.

 

[LRRPF52]

About 15 years ago, I ran into a gunsmith at a local shooting range who had squared and trued-up everything he could think of for a Grendel build.

He squared the barrel extension, the receiver/extension/barrel nut faces and bearing surfaces, trued-up/cut the threads himself, and de-edged/blended/polished his feed ramps.

He was shooting .5” groups out of the gate with it. I think he had a 22” barrel of one of the pedigreed barrel shops, maybe a Shilen.

He’s where I got the idea for the barrel extension work I do for reliability/feeding/babying brass.

As to the question about bolt carriers, most people don’t realize the bolt carrier actually rides high inside the raceway due to cartridge stack upward pressure from the magazine spring. The rails-in-contact are the top rails on the carrier, while the bottom rails kinda bounce off the inner lower raceway track. The initial contact area for the bottom of the carrier is the center spine at 6 o’clock that rides on top of the cartridge in pre-presentation.

Once the carrier clears the cartridge stack, the bottom rails are free to fall into-position with gravity, then they lift above again once the bottom spine rides over the next cartridge in the magazine and the stack lifts the carrier back up into tension against the inner top of the raceway.

What 45-90 is doing with the 4 hemisphere bolts on the carrier tail is controlling some of that banging around that happens normally with AR-15s, centering the carrier tail throughout its travel inside the RET, if I’m interpreting his placement of those bolts correctly ( I might be totally off my rocker as well.)

 

[357Max]

If reducing bolt rock or improving alignment is a goal, you can buy a carrier with a larger tail.

That's one of the reasons I run Sharps carriers. Sharps small frame carriers are +.030 on the tail. Have not confirmed if same applies to the large frame carriers.

 

[45-90]

About 15 years ago, I ran into a gunsmith at a local shooting range who had squared and trued-up everything he could think of for a Grendel build.

He squared the barrel extension, the receiver/extension/barrel nut faces and bearing surfaces, trued-up/cut the threads himself, and de-edged/blended/polished his feed ramps.

He was shooting .5” groups out of the gate with it. I think he had a 22” barrel of one of the pedigreed barrel shops, maybe a Shilen.

He’s where I got the idea for the barrel extension work I do for reliability/feeding/babying brass.

As to the question about bolt carriers, most people don’t realize the bolt carrier actually rides high inside the raceway due to cartridge stack upward pressure from the magazine spring. The rails-in-contact are the top rails on the carrier, while the bottom rails kinda bounce off the inner lower raceway track. The initial contact area for the bottom of the carrier is the center spine at 6 o’clock that rides on top of the cartridge in pre-presentation.

Once the carrier clears the cartridge stack, the bottom rails are free to fall into-position with gravity, then they lift above again once the bottom spine rides over the next cartridge in the magazine and the stack lifts the carrier back up into tension against the inner top of the raceway.

What 45-90 is doing with the 4 hemisphere bolts on the carrier tail is controlling some of that banging around that happens normally with AR-15s, centering the carrier tail throughout its travel inside the RET, if I’m interpreting his placement of those bolts correctly ( I might be totally off my rocker as well.)

Basically one follows in the footsteps of those armorers willing to discuss, in print or video, the things they do to make match winning ARs.
Then one adds, subtracts, and mixes their findings into what is learned.
It's a never ending endevor, always learning.

Here is a visual difference...Same match ammo, same day...2 rifles
the top group is a factory Areo Precision upper fully assembled by them...
Bottom is first 5 shots fired out of a new Proof barrel assembled in an upper with all the gunsmith accuracy enhancements listed.. with 5 into half inch right out of the gate, and is to be expected.
This is normal accuracy, with the full process, and a quality barrel...5 shots will hit .25" to .6" with a variety of ammo...and sometimes LC brass with cheap varmint bullets will shoot .3" & .4" 5 shot groups...so it doesn't always take match ammo to be accurate.

Yes, it all makes a difference. Unfortunately some of it requires machine tools, and specialized equipment, but do everything you can with hand tools and glue in or shims to improve accuracy.
There are other things never discussed but were taught, that do not require machine tools to improve accuracy.
I do not use them in my build/ assembly, but others might. ARs are accurate.

 

[45-90]

The barrel extension spigot


Odds are that even after truing the face the bore is conical. It's pretty unlikely the face looks like that and the bore is strait.

The bore is straight... a precision ground rod passes through, and is used to make the face perpendicular to the bore...a .001" finish cut is made to clean up all around.
It was stopped and photographed to show the that CNC machining is not perfection in much of today's production...
This is an example of bad programming, and untrained operators running them.
And depending on deburring machines to cover up slight imperfections, so the average guy doesn't know, as the anodized finish looks pretty good to the untrained eye.

 

[610yd]

My method is a bit odd to most, but I have achieved very good results while maintaining serviceability.
The point of the effort is to help reduce creep when it heats. This means making sure the nose of receiver is flat, and, the barrel extension is tight in the bore. We cannot really achieve a perfect fit, thus use a goop to fill in the micro gaps and hold tight.
The green 680 is ok, need to watch temps, and, pita to undo. I use a hi-temp red silicone product for general rifle stuff, it lives well with the temps and it doesn't really squeeze all that much, so it holds barrel tight when heat starts to cause expansion of the metals. I basically just apply a light smear to ID of the receiver, OD of the extension, light twisting as barrel gets installed, wipe/mop/qtip/clean residual goop from barrel and from inside receiver, I then set barrel nut to approx 50% of torque (lube the thread), let that set for 24hrs, come back and finish torque on nut. When thin layer of silicone cures, it's still fairly difficult to pull apart, but much easier than any hard set stuff.

All out precision/accuracy, something better than a hi-temp silicone is needed, but I am not convinced a super hard-set is the answer. I know BCM undercuts the barrel extension so you need to heat the receiver and cool the barrel, it's just tricky to install and lock up the nut before the interference fit grabs tight, kinda need a helping hand to do it quickly. I don't like interference fitment here because it can cause non-desired stresses, granted the aluminum will expand faster than the steel as temps go up, but that also means it's possible for the interference fit to become non-interference fitment.

 

[WeR0206]

My method is a bit odd to most, but I have achieved very good results while maintaining serviceability.
The point of the effort is to help reduce creep when it heats. This means making sure the nose of receiver is flat, and, the barrel extension is tight in the bore. We cannot really achieve a perfect fit, thus use a goop to fill in the micro gaps and hold tight.
The green 680 is ok, need to watch temps, and, pita to undo. I use a hi-temp red silicone product for general rifle stuff, it lives well with the temps and it doesn't really squeeze all that much, so it holds barrel tight when heat starts to cause expansion of the metals. I basically just apply a light smear to ID of the receiver, OD of the extension, light twisting as barrel gets installed, wipe/mop/qtip/clean residual goop from barrel and from inside receiver, I then set barrel nut to approx 50% of torque (lube the thread), let that set for 24hrs, come back and finish torque on nut. When thin layer of silicone cures, it's still fairly difficult to pull apart, but much easier than any hard set stuff.

All out precision/accuracy, something better than a hi-temp silicone is needed, but I am not convinced a super hard-set is the answer. I know BCM undercuts the barrel extension so you need to heat the receiver and cool the barrel, it's just tricky to install and lock up the nut before the interference fit grabs tight, kinda need a helping hand to do it quickly. I don't like interference fitment here because it can cause non-desired stresses, granted the aluminum will expand faster than the steel as temps go up, but that also means it's possible for the interference fit to become non-interference fitment.

Do you happen to have a link for the silicone product you use?

Something like this?

https://a.co/d/ejd9SAq

or this?

https://a.co/d/e2fYRJD

 

[610yd]

Do you happen to have a link for the silicone product you use?

JB Weld Red Hi Temp --> https://www.jbweld.com/product/hi-temp-red-silicone
I did forget to mention, after the smear it's good to lightly spritz (mist is probably a better word) the barrel end with water because the H2O helps cure the silicone in the tight space. Without the spritz it's almost like transferring silicone from one container to another and it may never fully cure.

There are automotive silicone products too that cure a bit stiffer, but they don't have the needed hi temp rating.

 

[WeR0206]

JB Weld Red Hi Temp --> https://www.jbweld.com/product/hi-temp-red-silicone
I did forget to mention, after the smear it's good to lightly spritz (mist is probably a better word) the barrel end with water because the H2O helps cure the silicone in the tight space. Without the spritz it's almost like transferring silicone from one container to another and it may never fully cure.

There are automotive silicone products too that cure a bit stiffer, but they don't have the needed hi temp rating.

Thanks! Ill add it to the inventory.

 

[MikeMiller]

JB Weld Red Hi Temp --> https://www.jbweld.com/product/hi-temp-red-silicone
I did forget to mention, after the smear it's good to lightly spritz (mist is probably a better word) the barrel end with water because the H2O helps cure the silicone in the tight space. Without the spritz it's almost like transferring silicone from one container to another and it may never fully cure.

There are automotive silicone products too that cure a bit stiffer, but they don't have the needed hi temp rating.

I have never tried that. Thanks for tip. Been using the Loctite bearing mount

 

[striped1]

You can do all of that stuff if it gives you more confidence in your build but if you're test-fitting your barrel into your upper and it's rattling around (and assuming that your barrel extension is not somehow undersized), I'd be looking for a better quality upper receiver.

It isn't always a low quality upper receiver. I can just be tolerence stacking between the receiver and the barrel. If I am making a 1000 ARs I can try multiple barrel / receiver combinations to get the best fit. Building one, I cannot, thus I shim and I send to a machine shop to lap.

 

[Lightness]

Doesn't it make more sense to use aluminum shims than steel? Matched rate of thermal expansion would be a benefit filling the space left by the isotropic expansion of the upper.

 

[WeR0206]

JB Weld Red Hi Temp --> https://www.jbweld.com/product/hi-temp-red-silicone
I did forget to mention, after the smear it's good to lightly spritz (mist is probably a better word) the barrel end with water because the H2O helps cure the silicone in the tight space. Without the spritz it's almost like transferring silicone from one container to another and it may never fully cure.

There are automotive silicone products too that cure a bit stiffer, but they don't have the needed hi temp rating.

Do you apply any kind of release wax/agent on the inside of the upper receiver before getting any JB on it?

 

[45-90]

Do you apply any kind of release wax/agent on the inside of the upper receiver before getting any JB on it?

I use Johnsons paste wax to the upper's bore, where the barrel extension fits into, and buff ...leaves a slick finish for disassemblely.

To remove:
Get an oak dowel from home depot to make the high temperature bering sleeve loctite easy to remove.
Turn down or sand the front of the dowel to hit the back of the barrel extension.
Leave it about 18" long or a bit more.
Slide the dowel up the carrier raceway against the back of the barrel extension...and smack the back of the dowel fairly hard on the concrete floor...hang on to the barrel! Notice as it seperates tap lighter.
Barrel is out in a few seconds.

 

[610yd]

Do you apply any kind of release wax/agent on the inside of the upper receiver before getting any JB on it?

Just for clarity, it's JB Silicone, not any epoxy stuff.

Using a Silicone product, no release of any kind. I clean the surfaces with rubbing alcohol. You want the Silicone to latch on (stick), which helps reduce movement. The thin layer of Silicone will still shear loose when you pull barrel out, but it requires a good pull (no heating or anything), but nothing like using a 680. Think of Silicone like head gaskets that have a silicone layer on them, the thin layer compresses just a smidge, but it also starts to act like a incompressible fluid when it cannot squish out.

And yep, when you remove the barrel you'll have some cleanup work to do, but it's very thin and can be buffed off rather easily.

 

[WeR0206]

Just for clarity, it's JB Silicone, not any epoxy stuff.

Using a Silicone product, no release of any kind. I clean the surfaces with rubbing alcohol. You want the Silicone to latch on (stick), which helps reduce movement. The thin layer of Silicone will still shear loose when you pull barrel out, but it requires a good pull (no heating or anything), but nothing like using a 680. Think of Silicone like head gaskets that have a silicone layer on them, the thin layer compresses just a smidge, but it also starts to act like a incompressible fluid when it cannot squish out.

And yep, when you remove the barrel you'll have some cleanup work to do, but it's very thin and can be buffed off rather easily.

Ah yes good point. I do think some folks bed with the epoxy stuff but I don't want to mess with that. As you noted silicon is much easier to work with/remove/etc. Thanks again for all the tips!

 

[Cascade Hemi]

Going back to a comment I made earlier, Indian Head shellac works really well. 99% alcohol disolves it and it's easy to work with. There's no need to reinvent the wheel.

 

[supercorndogs]

Just for clarity, it's JB Silicone, not any epoxy stuff.

Using a Silicone product, no release of any kind. I clean the surfaces with rubbing alcohol. You want the Silicone to latch on (stick), which helps reduce movement. The thin layer of Silicone will still shear loose when you pull barrel out, but it requires a good pull (no heating or anything), but nothing like using a 680. Think of Silicone like head gaskets that have a silicone layer on them, the thin layer compresses just a smidge, but it also starts to act like a incompressible fluid when it cannot squish out.

And yep, when you remove the barrel you'll have some cleanup work to do, but it's very thin and can be buffed off rather easily.

Head gaskets with a silicone layer on them???

 

[WeR0206]

Going back to a comment I made earlier, Indian Head shellac works really well. 99% alcohol disolves it and it's easy to work with. There's no need to reinvent the wheel.

Thanks for the reminder. I have some loctite 620 on hand but I think if I need to bed a barrel in the future I will use one of the easier to remove substances like high temp silicone sealant or Indian Head Gasket Shellac (removable with iso alcohol). This thread is a wealth of knowledge!

 

[610yd]

Head gaskets with a silicone layer on them???

Not putting silicone on them, they are made with a silcone layer, etc.

Thanks for the reminder. I have some loctite 620 on hand but I think if I need to bed a barrel in the future I will use one of the easier to remove substances like high temp silicone sealant or Indian Head Gasket Shellac (removable with iso alcohol). This thread is a wealth of knowledge!

Experiment with that Permatex before use, that stuff shrinks up quite a bit, and, if the ends seal up good from air contact it's possible you formed another container and the middle section can't cure out. It's rated 350F. Dab a blob on piece of glass, take pic, let it out-gas for 24hr, then look at it and compare.

Also look at Permatex Aviation Form-a-Gasket, seems to have rating of 400F. The Aviation and Indian stuff is similar when looking at their datasheets. The dried goop never really hardens hard.

Using an epoxy is also difficult to use because of their low service temps.

The JB Red is rated much higher. Temp was the hardest thing to find in usable goop.

 

[WeR0206]

Not putting silicone on them, they are made with a silcone layer, etc.





Experiment with that Permatex before use, that stuff shrinks up quite a bit, and, if the ends seal up good from air contact it's possible you formed another container and the middle section can't cure out. It's rated 350F. Dab a blob on piece of glass, take pic, let it out-gas for 24hr, then look at it and compare.

Also look at Permatex Aviation Form-a-Gasket, seems to have rating of 400F. The Aviation and Indian stuff is similar when looking at their datasheets. The dried goop never really hardens hard.

Using an epoxy is also difficult to use because of their low service temps.

The JB Red is rated much higher. Temp was the hardest thing to find in usable goop.

I think JB red high temp will be my go to for bedding going fwd. I will do the air test on the shellac you mentioned out of curiosity though. The directions on the shellac say to wait for it to air cure on the object for a few seconds first before assembling…probably to avoid the situation you described of it not curing due to being sealed from air.

Im curious if that high temp silicone would work well for sealing gas keys to carriers in lieu of loctite 620 or the permatex mil spec stuff?

 

[610yd]

Waiting for the shellac to dry before fitment may leave that loose-dry fitment with some voids. Dry shellac is also super sticky, kinda a pita to drag one surface against another where both have shellac, that goop is best for parts that come together like "clam-shell" (water pump, head gaskets, flange gaskets, etc). I assume you would coat ID and OD before install?

The choice of goop should be something that you can apply thick (I mention "thin layer", but that is thick relative to the small space between ID and OD of the fitment). The goop should be applied ID and OD, and it should ooze some requiring some cleanup (rubbing alcohol does clean-up silicone, I use the 90% alcohol), this "guarantees" the bedding is complete with no voids. Silicone is a good choice to do that, as are some other products, but it's the service temp of the product where we start to see limitations of choice. The actual use of the rifle matters. If your goal is to pop off one round every 30min for an intense 1000yd shot, then the lower temp goop stuff is probably ok. High rate semi-auto 16" 556 308, then you'll need something that's more hi-temp. Serviceability is also a build factor. If Low Temp and Low Serviceability are desired (likely yielding better accuracy and precision), then an epoxy is a good choice. In my small book of notes in bedding section, Temp and Serviceability are the two main factors.

Using JB Red for gas key sealing? Should work and seal ok. The thing to watch for is how using the goop changes final dimensions. You are basically wedging the goop between two mating parts, and that goop layer after bolting the two pieces together changes the location of the key (just a smidge). Think of it like adding anodize to bare aluminum, you change final dimensions. Silicone for gas key should be ok. You can also thin-down silicone as needed, and, there are many forms of silicone to be had, from toothpaste to very fluid viscosity.

For all out data, you can get some empirical data by clamping goop between two flange plates, apply a Torque to the lubricated bolts, let it cure, then mic the thickness of the sandwich at some defined location. Repeat with various goop. High solids goop end up thicker, etc.

 

[WeR0206]

don't like interference fitment here because it can cause non-desired stresses, granted the aluminum will expand faster than the steel as temps go up, but that also means it's possible for the interference fit to become non-interference fitment.

With what you pointed out here ^^^ re: the possibility for interference fits to become non interference once the chamber heats up, would you still use a shim or bed the barrel with high temp silicone when installing on a thermofit upper?

 

[610yd]

Interference fitments are sometimes hard to control, it all kinda depends on how much interference there is at room temp. MIC'ing the two within say 0.0005" and checking how round they are can be an indicator as to if it comes loose once it gets hot. Need some real lab testing to know for sure, or to determine how much interference is needed to avoid the loose situation when it gets hot. I think you would need a ton of heating and chilling to make the gap big enough to use goop for the fitment to mate. There some basic calculations (using online free calculators) to see how much the aluminum and steel will change size as they get hotter. It's a good estimate. My beef with hard interference fitments in this specific application, the aluminum is kinda weak in that area, so I would avoid adding stress when I can.

If we go a step further, for reg slip fitments, if you heat both sides to say a low 200F and then do the fitment with goop, lock up the nut to 50%, and then keep the whole thing in the over at 200F for about 24hrs, then you turn off the oven and allow it to cool down on its own for 24hrs, the ID side will then start to clamp down on the OD due to expansion coefficient diffs, a sandwich clamp with goop in the middle. When it all starts to heat up again it will return back to about the same point when it was installed at 200F.

Actual results from all this bedding chatter on accuracy-&-precision are not well documented. I think most bedding techniques suffice to the point where ammo and shooter have enough variance in them to account for where the bullet hits, etc. I just happen to find the silicone method does help stabilize the fitment, and, it's not a big pita to take apart.

And tidbit - that shellac stuff also softens when it heats up. It shines as a sealer due to it being sticky.

 

[WeR0206]

Interference fitments are sometimes hard to control, it all kinda depends on how much interference there is at room temp. MIC'ing the two within say 0.0005" and checking how round they are can be an indicator as to if it comes loose once it gets hot. Need some real lab testing to know for sure, or to determine how much interference is needed to avoid the loose situation when it gets hot. I think you would need a ton of heating and chilling to make the gap big enough to use goop for the fitment to mate. There some basic calculations (using online free calculators) to see how much the aluminum and steel will change size as they get hotter. It's a good estimate. My beef with hard interference fitments in this specific application, the aluminum is kinda weak in that area, so I would avoid adding stress when I can.

If we go a step further, for reg slip fitments, if you heat both sides to say a low 200F and then do the fitment with goop, lock up the nut to 50%, and then keep the whole thing in the over at 200F for about 24hrs, then you turn off the oven and allow it to cool down on its own for 24hrs, the ID side will then start to clamp down on the OD due to expansion coefficient diffs, a sandwich clamp with goop in the middle. When it all starts to heat up again it will return back to about the same point when it was installed at 200F.

Actual results from all this bedding chatter on accuracy-&-precision are not well documented. I think most bedding techniques suffice to the point where ammo and shooter have enough variance in them to account for where the bullet hits, etc. I just happen to find the silicone method does help stabilize the fitment, and, it's not a big pita to take apart.

And tidbit - that shellac stuff also softens when it heats up. It shines as a sealer due to it being sticky.

Makes sense. Thanks again for all the tips!

 

[PappyM3]

One thing to consider with the thermal fitment though, is that for installation you are heating up the aluminum first from the outside. During a shot string, the steel barrel extension is going to be heated first from the inside. Yes, the aluminum will heat up quicker once the heat gets to it, but the question is how hot will the barrel extension already be by the time the aluminum starts to heat up.

The next question is how hot does a barrel extension get? We know the chamber and forward get really hot, along with the gas tube. But has anyone measured the temperature of the barrel extension itself after strings of fire?

Last question, during an intense shot string, do you really care about a 10% reduction of accuracy due to the lack of a thermal fit barrel? Once you go back to a normal cadence of fire, the thermal fitment will be there again.

 

[610yd]

Hi @PappyM3
A quality metric of the build is repeatable results across all of the operating conditions. If I wanted to get all techy with it, I would embed a thermo-couple into the barrel extension and using a micro-controller capable optic and I would code in reticle adjustments based on temp. The actual adjusting would 1st be documented, then converted into code for the micro-controller. But our small arms rifles, at least the ones we use, are not that sophisticated just yet.

We are playing on the Pros-&-Cons field. With the interference fitment the aluminum will sink heat out of the steel, but a slip fitment with silicone (or other) will have an insulator between the two, albeit a thin layer.

Without getting into the technical weeds, we can obtain better repeatable accuracy and precision by adding movement resistance, which means using some goop to fill in the micro gaps between barrel and receiver. And certainly yes, even the goop changes properties as it warms up (hardness, expansion, etc).

The only gripe I have with interference fitment is that you are adding stress when it all finally reaches same temp, and the aluminum ID does not have symmetrical aluminum around it, so when that ID expands from heating it is not doing so evenly (the bore is not a simple ring). Because of this I prefer the goop method.

For AR, think about what might be if the receiver bore and bbl extension were just 2x longer. At least on the AR platform I suspect the .mil design was just enough for the hardware to be better than ammo+shooter. I am actually surprised no AR maker has tried that yet. Aero has tried some odd stuff, like M4E1 Enhanced upper (which is nice), but nothing yet for a longer receiver bbl bore.

 

[45-90]

Just add a ground carbide ring around the barrel extension...it does not transfer heat well.
Brownells heat stop paste works good.
Forced liquid cooling?
I don't think it's that important, but it's all there for some to try.

I'm satisfied with the green high temperature bering sleeve loctite for most applications.
Does it have any heat barrier properties being so thin? Don't know.
But it's easy and works good enough, also easy to disassemble when done right...
Plus along with other accuracy enhancements tends to improve accuracy...the AR 15 is what it is.
It has a lot of limitations because of its design, a light battle rifle in 5.56 cal.
But good accuracy can be had with modifications, trueing, quality barrel, etc...then ya want to run it red hot with mag dumps. Those two concepts do not go together well in one rifle...but the AR 15 does well IMO bridging those two concepts.

 

[912173]

I've had good results with regular jb weld so far. It says it's good to 550 F. I used imperial sizing wax for release agent, so it should come apart if needed.

 

[610yd]

What rated temp is sizing wax? If it goes on 0.0005" film and it "burns out", you now have a 0.002" gap in there (1thou on OD + 1thou on ID). Or, does the wax mix in with the goop during install?

We must also account for how close to max temp you get. As example, if epoxy is rated 250F and the setup runs near 200F, you are much closer to max temp of that material, although it's not failing from temp. With silicone rated 500F you would have a lot more reserve from that material.

If I were to choose an epoxy like JB I would use the JB Red Silicone as the release material, very lightly (very thin, like rub in with finger like an oil coating) coating the ID and OD, mist it with water, let it cure a few hours, then use the epoxy. My only issue with a hard epoxy is it can get into areas you dont want it. A smidge of silicone on the nut threads is not an issue, it wont seize the nut.

In my experience, albeit perhaps not as robust as others, I have not seen any discernible diff between just using the silicone vs waxing and using an epoxy, the ammo and shooter always seem to be the main factors in "bad" shooting when it comes down to rifle accuracy and precision. However, I do notice it's easier to pull apart the silicone-only setup.

I guess you choose wisely. As example, if the dry slip-fit is super sloppy, then maybe silicone is not the best choice.

I my own experience, starting with something that is not a sloppy dry fit, the hi-temp silicone method seems to work a-ok for resisting bbl movement, and, being able to pull it apart. I think in the end we'll find most errors are coming from ammo, shooter hands, and defects in barrel bore. The bedding is helping to reduce overall error, yet it's not as significant as the other three mentioned. We probably need 100x more effort to make ammo that is on same magnitude of error correcting that we obtain from a bedding procedure.

JB also has something called JB ExtremeHeat rated 1000F. I have experimented with similar but the product was very gritty and not usable for mating parts in bbl/receiver application. JB Weld I believe is intermittent rated 550F, but that is perhaps good enough. In the material sheet, parts A & B have flashpoint around 390F or less. The Ai answer for JB Weld says rated 500F and intermittent 600F. I have my reservations on that claim.

 

[supercorndogs]

It's not hard to knock out a barrel glued in with green loctite. I woukd trust jb with .002 clearance to stiffen the connection more than silicone. I trust green loctite bearing retaining compound more.

I do make sure I don't have a problem child barrel before gluing, but a little heat and and oak dowl they come back out with out much fuss.

 

[N_Turs]

It's not hard to knock out a barrel glued in with green loctite. I woukd trust jb with .002 clearance to stiffen the connection more than silicone. I trust green loctite bearing retaining compound more.

I do make sure I don't have a problem child barrel before gluing, but a little heat and and oak dowl they come back out with out much fuss.

This is exactly what I do, green loctite and a wood stick for removal. I've done a bunch like this and removed a few here and there they come out just fine.

 

[610yd]

The 2thou" I mentioned is air space that would happen if some goop (wax) on ID and OD is just 1/2thou" thick and that goop disappears somehow. That 1/2 thou is on the radius of ID and OD, so it's x2 (dia) for ID and OD , where ID gets bigger and OD gets smaller, so the net sum is x4, or 0.002".

Most goops don't just vanish though.

620 does carry decent temp ratings, the Henkel tech doc shows it to about 250C 482F(max) and holds about 68% strength at that temp, and holds 100% at 170C 338F. It actually achieves more than 100%(@28C) between 29C to 170C, weird but that's what we get from chemicals.

We know the aluminum grows with heat, so we would like the goop to do the same.
Coefficient of Thermal Expansion (x10^-6)
620 - 80
Silicone - is near 200
6061/7075 - 24

Although the goop may expand more by CoE than aluminum as temps rise, the goops' duro hardness goes down, less resistance to movement. Since goop layer is magnitudes smaller than the aluminum and steel, the net growth of goop expansion does not keep up with the metals.

With the JB Red silicone, I don't need a dowel or do any banging on anything, I can just secure receiver and pull out the barrel with a little muscle, the thin silicone will shear with "small" force, from there things clean-up easily. I don't get that experience from using 620 or waxed epoxy. 620 and epoxy have high strength in both shear and tensile, perhaps a wax release layer with epoxy lessens the force needed.


Silicone, 620, waxed epoxy, just epoxy, are all valid choices, but each has Pros & Cons. I'll bet on 620 and epoxy holding bbl tighter, but in same bet I bet ya the overall gains from the bedding diffs (epoxy vs silicone, 620 vs epoxy, etc) are lost in ammo/shooter/bbl.

Note: if I previously mentioned the loctite 680, I did mean 620. 680(green) is a lower viscosity item and it does not have good temp chart like 620 does.

 

[supercorndogs]

The 2thou" I mentioned is air space that would happen if some goop (wax) on ID and OD is just 1/2thou" thick and that goop disappears somehow. That 1/2 thou is on the radius of ID and OD, so it's x2 (dia) for ID and OD , where ID gets bigger and OD gets smaller, so the net sum is x4, or 0.002".

Most goops don't just vanish though.

620 does carry decent temp ratings, the Henkel tech doc shows it to about 250C 482F(max) and holds about 68% strength at that temp, and holds 100% at 170C 338F. It actually achieves more than 100%(@28C) between 29C to 170C, weird but that's what we get from chemicals.

We know the aluminum grows with heat, so we would like the goop to do the same.
Coefficient of Thermal Expansion (x10^-6)
620 - 80
Silicone - is near 200
6061/7075 - 24

Although the goop may expand more by CoE than aluminum as temps rise, the goops' duro hardness goes down, less resistance to movement. Since goop layer is magnitudes smaller than the aluminum and steel, the net growth of goop expansion does not keep up with the metals.

With the JB Red silicone, I don't need a dowel or do any banging on anything, I can just secure receiver and pull out the barrel with a little muscle, the thin silicone will shear with "small" force, from there things clean-up easily. I don't get that experience from using 620 or waxed epoxy. 620 and epoxy have high strength in both shear and tensile, perhaps a wax release layer with epoxy lessens the force needed.


Silicone, 620, waxed epoxy, just epoxy, are all valid choices, but each has Pros & Cons. I'll bet on 620 and epoxy holding bbl tighter, but in same bet I bet ya the overall gains from the bedding diffs (epoxy vs silicone, 620 vs epoxy, etc) are lost in ammo/shooter/bbl.

Note: if I previously mentioned the loctite 680, I did mean 620. 680(green) is a lower viscosity item and it does not have good temp chart like 620 does.

So this reads like you have tried every method mentioned. Is that what you are asserting? What's your average group size for a silicone bedded barrel vs locktite vs epoxy bedding?

 

[45-90]

Lots of speculation, misinformation ...but no actual measurements.
I did the Johnsons Floor Wax for a release agent and put on a coat part way down the reciever extension bore.
Then buffed it after drying.
Measured with a 10 thousandths indicator.
No movement of the dial not even .0001" thick. That's not even 1 ten thousandth thickness added...no discernable measurement...so it does not interfere with any fitment. If it was to melt away, no measurable difference. If it hangs in there it helps with removal ...as a slick surface.

Green 620 Loctite high temp bearing sleeve , is not only good for high heat applications...but "expands" as it sets up to fill voids up to .007" ...according to the manufacturer.

Which is good because a larger problem is the reciever extension bore is likely slightly out of round.
A half to one thousandths of an inch out of round in many cases.
The one I just measured is close to a full .001" out of round.
The extension is ground but is then threaded on, and torqued down... and it too is most likely not running true ...I have noticed this before, when rechambering AR barrels.

The Wax thickness test ...nothing measurable with a 1 10 thousandths indicator.
This is the product used.

Change to a half thousandths indicator and measure the out of roundness.
This reciever bore is close to .001" out of round, 3 sides being pretty close to zero, but one aide goes slightly high, about .0009".
It is what it is...those are the facts I found when actually measuring.
Do whatever works for you ..
I'll keep doing the same procedure until I find something better.

 

[WeR0206]

The 2thou" I mentioned is air space that would happen if some goop (wax) on ID and OD is just 1/2thou" thick and that goop disappears somehow. That 1/2 thou is on the radius of ID and OD, so it's x2 (dia) for ID and OD , where ID gets bigger and OD gets smaller, so the net sum is x4, or 0.002".

Most goops don't just vanish though.

620 does carry decent temp ratings, the Henkel tech doc shows it to about 250C 482F(max) and holds about 68% strength at that temp, and holds 100% at 170C 338F. It actually achieves more than 100%(@28C) between 29C to 170C, weird but that's what we get from chemicals.

We know the aluminum grows with heat, so we would like the goop to do the same.
Coefficient of Thermal Expansion (x10^-6)
620 - 80
Silicone - is near 200
6061/7075 - 24

Although the goop may expand more by CoE than aluminum as temps rise, the goops' duro hardness goes down, less resistance to movement. Since goop layer is magnitudes smaller than the aluminum and steel, the net growth of goop expansion does not keep up with the metals.

With the JB Red silicone, I don't need a dowel or do any banging on anything, I can just secure receiver and pull out the barrel with a little muscle, the thin silicone will shear with "small" force, from there things clean-up easily. I don't get that experience from using 620 or waxed epoxy. 620 and epoxy have high strength in both shear and tensile, perhaps a wax release layer with epoxy lessens the force needed.


Silicone, 620, waxed epoxy, just epoxy, are all valid choices, but each has Pros & Cons. I'll bet on 620 and epoxy holding bbl tighter, but in same bet I bet ya the overall gains from the bedding diffs (epoxy vs silicone, 620 vs epoxy, etc) are lost in ammo/shooter/bbl.

Note: if I previously mentioned the loctite 680, I did mean 620. 680(green) is a lower viscosity item and it does not have good temp chart like 620 does.

Slightly off topic but have you used the high temp silicone to help seal around the front/back of a loose fitting gas block and possibly also seal where the gas tube meets the gas block? I know folks have used the 620 to seal around gas blocks so im curious if you’ve used the silicone in a similar way.

 

[610yd]

Yep, I have used JB Red silicone to seal up GB to bbl, and tube to GB. It's a bit tricky so not to get the goop into the ports.
For GB you need to leave a gap (no goop) in front of the barrel port, place thin ring of goop behind the port, and the ring (with gap) in front, then slide GB straight on, align, lock. After GB lockdown you can try and wedge a smidge of goop into the corner of GB where the gap was, then wipe up the excess. For the tube, insert into alignment, pull it back about 1/8", goop the tube on back side of GB, then push tube in so it extends past the front of the GB by about 1/16", goop that part of the tube, then pull it back into alignment and pin, wipe off the excess goop. I do place the goop on barrel near where the edges of the GB will end up. Obviously the front side goop is gonna smear back as you slide the GB straight on (no twisting).

Caveat for gooping the tube. When it's just pinned (no goop) it will have some movement in it so that the tube can easily align through the receiver and to BCG. With goop you should do the full install (tube to GB, GB onto barrel) then check with BCG that tube alignment is good, then leave BCG in there while the goop sets up overnight.

Method may vary depending on GB used, etc. And although I have used GB alignment jigs for dimpling, I resort to using bore scope to verify gas port alignment before locking down the GB.

 

[610yd]

Green 620 Loctite high temp bearing sleeve , is not only good for high heat applications...but "expands" as it sets up to fill voids up to .007" ...according to the manufacturer.

Which is good because a larger problem is the reciever extension bore is likely slightly out of round.
A half to one thousandths of an inch out of round in many cases.

I am not speculating. It's more about ideal vs non-ideal, physics.

Expanding goop is not really desired, but I assume it will try and squish out as it expands in curing. Being a liquid or paste alone is enough to fill the space, and it's certainly non-desirable to use a goop that shrinks while curing.

From technical weeds page, if there's 1 or 3 or 5 thou localized space in there and the rest is an even 0.5thou, that space is gonna provide a little more localized stress in fitment as the 620 expands with heat. Not ideal, but likely not causing a notable diff in accuracy or precision.

As for waxing and buffing, too much work (for me). You can just finger rub the ID and OD with Red silicone, it will leave same very thin layer and acts as a release layer, mist it, let sit 1hr, goop & install.

I agree whatever works for the installer. All the methods work, Pros & Cons for each. And to boot, the diffs between method-A vs method-B will be lost in ammo/shooter/barrel.

As we know, zillion AR's have been made by slapping parts together w/o all this techy stuff, and they'll shoot ok with crappy boxed ammo. 2moa lego AR vs the well built 1/2moa, yes, but also way more money and effort to do 1/2moa. I probably not even choosing AR for high accuracy & precision.

My conclusion is, facing and bedding is desired, actual bedding method used does not matter all that much. I choose goop that is temp friendly, easy cleanup, less worry about spill-over, and less effort on the serviceable side (no hammer needed).

 

[610yd]

One thing I missed about 620, is that the substrate matters.
620 on mating aluminum, it only gets to about 50% of full rated strength.
620 is designed to achieve the 100% rated strength on clean steel mating parts.

So I wonder what the final cure strength of 620 is when the mating surfaces are waxed, or just OD steel and ID anodize? Not that a lower cure strength makes a big diff here, but still curious.

 

[PappyM3]

One thing I missed about 620, is that the substrate matters.
620 on mating aluminum, it only gets to about 50% of full rated strength.
620 is designed to achieve the 100% rated strength on clean steel mating parts.

So I wonder what the final cure strength of 620 is when the mating surfaces are waxed, or just OD steel and ID anodize? Not that a lower cure strength makes a big diff here, but still curious.

It cures just fine. When you pop out a barrel bedded with 620, you’ll notice that the hardened 620 stays on the steel barrel extension and is minimally on the aluminum receiver. So it adheres to the steel, but it fully cures.

 

[Ernest 5.56]

Just looking online you could get more strength on aluminum with a primer. LOCTITE® SF 7649

Haven't heard of anyone using it on a AR though




Henkel recommend the use of primers such as LOCTITE® SF 7649 & SF 7471 with most anaerobics where both parts are classified as "inactive" metals. These metals lack the presence of iron or copper content and thus, lack the "active" metal ions required to fully cure the anaerobic product in 24 hours. These metals include plated parts, stainless steel, galvanized steel and anodized aluminum, to name a few. If, on the other hand, both or at least one part is an "active" metal, such as plain steel, brass, bronze, copper or iron, there is no need to use a primer to ensure full cure in what is typically 24 hours at room temperature. A primer will also be helpful to accelerate the cure in colder temperatures or where there are larger gaps between the metal components.

 

[PappyM3]

Just looking online you could get more strength on aluminum with a primer. LOCTITE® SF 7649

Haven't heard of anyone using it on a AR though




Henkel recommend the use of primers such as LOCTITE® SF 7649 & SF 7471 with most anaerobics where both parts are classified as "inactive" metals. These metals lack the presence of iron or copper content and thus, lack the "active" metal ions required to fully cure the anaerobic product in 24 hours. These metals include plated parts, stainless steel, galvanized steel and anodized aluminum, to name a few. If, on the other hand, both or at least one part is an "active" metal, such as plain steel, brass, bronze, copper or iron, there is no need to use a primer to ensure full cure in what is typically 24 hours at room temperature. A primer will also be helpful to accelerate the cure in colder temperatures or where there are larger gaps between the metal components.

Good to know and thanks for the research, but I don’t really see a need to do that with AR bedding. The purpose isn’t to glue the barrel extension there, it’s just to fill the gaps so the barrel can’t move.

 

[610yd]

Like I said, Pros & Cons.

1) If you wax both ID and OD, then the substrate is wax and wax. Are people only doing a release wax on the ID? How then do you get the 620 off the barrel, or is the barrel now a throw away item?

2) 620 is expensive, has a short shelf life, and most won't use much of the tiny bottle.

3) The use of an activator is mostly for speeding up a "set" time, but it impacts strength of the 620 final cure. I have email somewhere from Henkel stating the activators in general reduce max strength of the anaerobics. The anaerobics work best with active substrates. This graph is similar to wax-wax substrate setup, be lucky to get 50% of what the 620 has to offer, which however may still be enough, maybe that's why it knocks out so easily?