AR15 operating system- low mass vs. balanced reciprocal mass advice

[Nivium]

Hide, I'm currently building out a grendel and looking advice on maximum recoil reduction. This thread is for those min/max'ers out there who understand the obsession with getting the absolute optimal results for a certain build purpose using every item available on the market.

My question is, for those with experience with both systems: does a low-weight operating system (titanium bcg, taccom buffer system, reduced power spring and low gas setting) always beat a heavier, but balanced operating system (normal bcg, hydraulic or H2/H3 heavy buffer, increased power spring and appropriately tuned gas)?

More specifically, what are some combinations that competition shooters are trying with success? I have heard of people pairing JP lowmass bcgs with heavy buffers and strong springs, so as to create more of a balance than a wall-to-wall reduction of reciprocating mass.

Based on my experience, I imagine that with a 233 a truly lightweight system like Taccom with super lightweight bcg beats a heavier system with a hydraulic buffer. Please let me know if you disagree, or if you think there will be a difference when applied to a heavier grendel.

Much appreciated

 

[Nivium]

No one try to go this far on recoil reduction?

 

[Yondering]

If you want good recoil reduction, put a good brake on it and tune the gas well. That beats any of the high end carrier/buffer setups if they're tuned poorly and don't have a brake.

Use a low mass carrier if you want, there are some advantages especially if you're just shooting one specific load, but go one way or the other. The people using low mass carriers with heavy buffers are just throwing money away to get more or less the same effect as a standard carrier and buffer setup.

The disadvantage of low mass setups is that they're more sensitive to different loads, while a heavy reciprocating mass can usually tolerate a wider range of loads. The big advantage to low mass setups is reduced movement of the rifle (not really recoil) as that mass slams back and forth. IME you'll notice the difference more when the carrier slams home in the forward direction, after the recoil, if you are able to tell the difference.

Also - this stuff has almost nothing to do with whether the rifle is a 223 or a Grendel, or any other cartridge. The action is operated by gas pressure and is not affected by recoil or bullet size/weight. In other words, a light carrier isn't any less appropriate for a Grendel than it is for a 223.

One final thing - if you're going to correctly tune the gas system, you don't need to also mess with different buffer weights. One or the other, no need for both.

 

[charnicus]

I’d recommend the way of full mass carrier, tunable gas, and dial in the buffer weight. No need to get wild.

 

[Nivium]

Also - this stuff has almost nothing to do with whether the rifle is a 223 or a Grendel, or any other cartridge. The action is operated by gas pressure and is not affected by recoil or bullet size/weight. In other words, a light carrier isn't any less appropriate for a Grendel than it is for a 223.

Thanks for the response. I should have mentioned that a muzzle brake is a given. I'm really trying to squeeze every bit of recoil reduction out of the system.

Good insight on 223 vs. Grendel, but do you think that applies notwithstanding that feed issues on a Grendel are pretty common? In other words, would the low mass system result in any difficulty stripping the grendel round, or require more gas pressure thus reducing the benefits of the low mass system, where that may not be an issue with a 223?

 

[357Max]

No one try to go this far on recoil reduction?

Don't get wrapped around the axle trying to figure out the best combo of the latest gimmicks.

And don't take this the wrong way, but your original post is focused on the wrong thing. Reliability was not mentioned & nothing beats it.

Remember the basics. Actual recoil happens when the bolt is locked. Only way to reduce that is with weight and an effective brake.
From there it's reciprocating mass. The platform was designed to run reliably, run dirty, & run select fire.
Most of the rooty kazooty gamer crap I see at the range would take a shit in the first 2 mags of full auto. Hell they often don't reliably function slow fire from a bench in 75 degrees dry weather.

I am a fan of a well designed adj gas block such as SLR and it is appropriate IMO to include on non std calibers.
The Mil and partner suppliers have spent a lot of time and money developing reliable buffer systems like the Vltor A5.
That is my go to starting point on a small frame AR. Then tune buffer/carrier weight if required, to have room in gas block adjustment.

Problems mostly result from non std non mil spec changes such as gas port size, gas system length vs barrel length etc. Combinations are endless.

As Yondering said, a Lt wt reciprocating system is a lot more finicky i.e. gun better be clean, don't change mag brands, bullet or charge weight etc.

Repeat after me................. there is no magic combination that'll make me shoot like Jerry Miculek. That would just take 100's of thousands of rounds dedicated to training.

I improve a lot quicker when I can 100% focus on my shooting vs what the fuck is wrong with this POS.

 

[Yondering]

Good insight on 223 vs. Grendel, but do you think that applies notwithstanding that feed issues on a Grendel are pretty common? In other words, would the low mass system result in any difficulty stripping the grendel round, or require more gas pressure thus reducing the benefits of the low mass system, where that may not be an issue with a 223?

It sounds like you're confusing two different operations. The gas system only makes the bcg move rearward, that's it. More gas = quicker unlocking and faster rearward travel of the bcg.

Forward travel of the bcg is determined by the spring force and to some extent the mass of the bcg and buffer combination. Low mass = faster forward travel. Heavy mass = slower travel but more momentum, and harder hit when it stops. The spring releases the same amount of energy either way.
Stripping a round from the magazine happens during this operation. Increasing gas to the carrier has no effect here, until you make it so overgassed that it's bouncing off the stop at the rear; don't do that.

The Grendel case size isn't particularly harder to strip from the magazine than a 5.56; that's not the cause of feeding issues. Even so, a light Ti carrier and light buffer have plenty of mass to strip the round from the mag. For example my 308 uses a modified carrier that is lighter than most AR15 carriers, and has no problem with feeding. The only way a light carrier can contribute to feed issues is if it's either not gassed appropriately, or it moves fast enough to outrun the magazine (like if you have a really light carrier but a heavy spring).

Spring force is the factor that could cause stripping issues; if you lighten the spring to the point that the carrier doesn't return with enough force to reliably strip a round, then you'll have feed issues. I have a light spring, buffer, and carrier setup for a subsonic-only 300 Blk; I don't have spring force numbers for you but have ended up with a spring several inches shorter than standard and it still strips rounds without issues. There is quite a bit of room to play and tune the AR15 system in every aspect, just change one thing at a time and try to understand what each change does for you.

It's good you're asking questions, but it helps a lot to understand the system and how each part functions. If you want low reciprocating weight (which does contribute to minimal rifle movement) go for it, just be prepared to tune it appropriately and don't assume it'll be ideal for a general purpose setup. Also don't be confused and mix/match heavy and light parts like some people do; figure out your goal and what compromises you're willing to make to achieve it, and then work toward that.

If you want a good general purpose setup though, you might be best off with a standard bcg and buffer, and just tune it well and use a good brake. That's how my 12.5" Grendel is set up, and it hardly moves at all; I have honestly connected with quick double taps on 18" steel plate at 600 yards from prone because the reticle moves so little during firing. That's even with an H2 buffer (I use a little more mass than necessary because of the different loads I shoot); I could lighten the mass further to make it track even flatter but it's not necessary.

 

[Constructor]

Reduce recoil for a game gun?
You've received advice about reducing recoil, here is a little about reliability. If it's a duty/defensive rifle and you want it to be reliable when it gets hot and dirty use a full weight carrier, heavy buffers and strong springs and tune the gas to run that setup. The momentum from the weight and the strong spring will ensure it can strip rounds from the mag and close the bolt when it gets hot, dirty and dry.

 

[Nivium]

It sounds like you're confusing two different operations. The gas system only makes the bcg move rearward, that's it. More gas = quicker unlocking and faster rearward travel of the bcg.

Forward travel of the bcg is determined by the spring force and to some extent the mass of the bcg and buffer combination. Low mass = faster forward travel. Heavy mass = slower travel but more momentum, and harder hit when it stops. The spring releases the same amount of energy either way.
Stripping a round from the magazine happens during this operation. Increasing gas to the carrier has no effect here, until you make it so overgassed that it's bouncing off the stop at the rear; don't do that.

The Grendel case size isn't particularly harder to strip from the magazine than a 5.56; that's not the cause of feeding issues. Even so, a light Ti carrier and light buffer have plenty of mass to strip the round from the mag. For example my 308 uses a modified carrier that is lighter than most AR15 carriers, and has no problem with feeding. The only way a light carrier can contribute to feed issues is if it's either not gassed appropriately, or it moves fast enough to outrun the magazine (like if you have a really light carrier but a heavy spring).

Spring force is the factor that could cause stripping issues; if you lighten the spring to the point that the carrier doesn't return with enough force to reliably strip a round, then you'll have feed issues. I have a light spring, buffer, and carrier setup for a subsonic-only 300 Blk; I don't have spring force numbers for you but have ended up with a spring several inches shorter than standard and it still strips rounds without issues. There is quite a bit of room to play and tune the AR15 system in every aspect, just change one thing at a time and try to understand what each change does for you.

It's good you're asking questions, but it helps a lot to understand the system and how each part functions. If you want low reciprocating weight (which does contribute to minimal rifle movement) go for it, just be prepared to tune it appropriately and don't assume it'll be ideal for a general purpose setup. Also don't be confused and mix/match heavy and light parts like some people do; figure out your goal and what compromises you're willing to make to achieve it, and then work toward that.

If you want a good general purpose setup though, you might be best off with a standard bcg and buffer, and just tune it well and use a good brake. That's how my 12.5" Grendel is set up, and it hardly moves at all; I have honestly connected with quick double taps on 18" steel plate at 600 yards from prone because the reticle moves so little during firing. That's even with an H2 buffer (I use a little more mass than necessary because of the different loads I shoot); I could lighten the mass further to make it track even flatter but it's not necessary.

This is the exact kind of answer I was looking for. Thanks a lot

You ever mess around with hydraulic buffers?

 

[Yondering]

This is the exact kind of answer I was looking for. Thanks a lot

You ever mess around with hydraulic buffers?

Glad to help.

Nope, hydraulic buffers are one thing I have not messed with. Partly because they are known to fail sooner or later, and partly because they seem to be a band-aid fix for an overgassed system.

 

[charnicus]

Start with gas and go from there. If you want to spend more than what’s typical for a buffer, get a JP SCS standard or H2 and a couple weights. Armaspec also makes a competitive alternative but I do not believe it is tunable. Haven’t really looked into that.

 

[LRRPF52]

OP started out with the statement that his goal is maximum recoil reduction, but there must be something else driving the primary intent of the rifle. What is that intent? Competition in action-based shooting sports? Hunting? SHTF?

I have always used full mass carriers, which follow the 3:1 mechanical engineering “rule” of carrier weight to bolt weight for reliable extraction and chambering. It’s one of the principles that Stoner and Jim Sullivan used on the AR-10 and AR-15.

Since 6.5 Grendel has factory loads ranging from 85gr to 130gr, you will find that there are gas system considerations to take into account especially if you have the gas dialed for 100-130gr, then try to shoot the little short 90gr TNT.

For me, I found that the 12” Grendel with .068” port, suppressed with TBAC Ultra 5, Bootleg carrier set to fully suppressed, will run 90gr TNT, 120gr Federal OTM, and 123gr Hornady just fine. Unsuppressed, fully-open gas, I had 2 out of 5 FTFeed malfs with 90gr TNT even with a more stiff action spring. Suppressed, it runs 90gr TNT like a raped ape even fully-choked.

If I try to shoot 90gr TNT in an 18” or 16” MLGS unsuppressed Grendel, it feels sluggish compared to all the other factory loads. I’m sure they’re using a faster-burning powder to punt the little 90gr TNT out at fast velocity, which behaves much differently than slower-burning powder behind a really long 120-130gr class bullet.

When I shoot hand-loaded 95gr Controlled Chaos, which is a solid, the 18” MLGS runs fine, very positive cyclic rate.

More important than how the gun feels when it cycles is keeping the bolt locked as long as possible.

Grendel doesn’t really recoil a lot as it is, so my perspective and opinion is to lean on reliability first, then mess with brakes and try to keep the BCG and action spring + buffer in a known-performance category.

Early unlocking with any gas gun is bad ju-ju for the bolt, brass, and moving parts in general.

If you ever shoot a Grendel with standard action parts and a half-decent brake, they don’t really move the reticle much at all on target. The longer the gas system, the smoother it will run. 20” RLGS Grendels are probably the smoothest.

 

[YammyMonkey]

A simple way to get a reliable & soft shooting gun would be to slap an A5 system on the rear, a standard FA BCG and a good adjustable gas block like an SLR. Start with the gas turned down, then add more until it'll reliably lock open. Add one click of gas to account for a dirty gun & go play.

If you're going to shoot a relatively narrow range of ammo, you can adjust to the weakest ammo & let the screw carbon lock so it won't come loose.

 

[Majja]

Well here is what I built up as a "game gun" Brownells Low mass BCG, Tapco Reduced power Spring light weight Short buffer and Adjustable GB then tuned for the softest load I shoot in the winter.

Ohh yea and a fortis RED brake.

I think it is funny how every one talks reliability and it has to be full mass. For a honest to god battle rife I kind of agree. But race guns have to be just as reliable, I just think it comes down to duty cycle. In a battle rifle I where the weapon is going to be drug through the mud, dirt and shot to hell and back and maybe cleaned once a deployment then yea over gas it on heavy springs with a full auto carrier.

My gun cycles soft fast and so far I have not had one malfunction. Although I have not been shooing as much as I would like either.

Find what works for you that is the fun of these guns they are easy to change.

 

[Yondering]

I have always used full mass carriers, which follow the 3:1 mechanical engineering “rule” of carrier weight to bolt weight for reliable extraction and chambering. It’s one of the principles that Stoner and Jim Sullivan used on the AR-10 and AR-15.

IME that "3:1 rule" is not very important or useful. I've gone way outside of that in both AR15 and LR308 platforms with no issues at all, as long as the gas system is tuned to match. And although I do keep my rifles well oiled, I don't clean them often, and the extra light carriers haven't been any more trouble there.

Low mass carriers can work fine and be reliable on a lot of AR setups, just make sure the gas is tuned to match.

 

[LRRPF52]

IME that "3:1 rule" is not very important or useful. I've gone way outside of that in both AR15 and LR308 platforms with no issues at all, as long as the gas system is tuned to match. And although I do keep my rifles well oiled, I don't clean them often, and the extra light carriers haven't been any more trouble there.

Low mass carriers can work fine and be reliable on a lot of AR setups, just make sure the gas is tuned to match.

For the military and hunting rifles, the 3:1 principle comes into play with cold weather. The extreme cold weather requirements for Mil-Std are the most demanding set of specs since the temperature range is so low, which asks a lot from a gas-operated weapon.

Since I live in a place with a pretty wide range of extremes (95-100˚ F in summer, down to -4˚ in the mountains in the winter), I don’t mess with low-mass carriers. I’ve shot them in the summer and temperate months here where they worked fine.

I’ve done a lot of high volume shooting in the Arctic and sub-arctic with various 5.56 barrel/gas system lengths as well, and full mass carriers work fine even down to -37˚C there, as long as the gun is set-up more like a TDP gun or better yet, Canadian Colt variants (better bolt).

 

[jbailey]

This is a great discussion and I'm learning a lot.

One question I had:
Is there any issues with running a carbine spring and buffer in a rifle extension tube, as long as you are using a buffer spacer in tube ???

I'm driving a 6ARC, rifle +2 gas system, an adjustable gas system, with several weight carbine buffers with a -10% and a +10% carbine springs, and can't find a reliable feeding combo. The buttstock is a Magpul UBR, rifle length tube.

 

[hlee]

@jbailey, what you are describing is essentially what you do with a JP Silent capture spring. It is a one size (physical length) fits all solution, with the addition of a spacer for rifle length extensions.

I have my y 224 predator set up with +2 gas, full mass carrier, adjustable gas block, and a SCS. I replaced the tungsten weights with steel weights (bought the heavy and needed the standard).

I would think you should be able to get a reliable running rifle with a standard weight buffer, full mass carrier, and an adjustable gas block.

 

[LRRPF52]

This is a great discussion and I'm learning a lot.

One question I had:
Is there any issues with running a carbine spring and buffer in a rifle extension tube, as long as you are using a buffer spacer in tube ???

I'm driving a 6ARC, rifle +2 gas system, an adjustable gas system, with several weight carbine buffers with a -10% and a +10% carbine springs, and can't find a reliable feeding combo. The buttstock is a Magpul UBR, rifle length tube.

As long as you put the buffer spacer in the Rifle RET, you can use a Carbine buffer and spring. That’s how the JP SCS works, using a spacer for rifle RETs.

What bullet weight are you shooting and in what kinds of temps?

What is your gas port diameter?

Did you start with the gas fully-open?

For a +2” ELGS, I don’t think I would mess around with choking the gas there because the port pressure will already be much lower than RLGS.

 

[jbailey]

As long as you put the buffer spacer in the Rifle RET, you can use a Carbine buffer and spring. That’s how the JP SCS works, using a spacer for rifle RETs. Buffer spacer is in.

What bullet weight are you shooting and in what kinds of temps?
95gr and 105gr. Temps are mid 50F. Powder is N540, RL15 and 2000MR. Mostly N540 however.

What is your gas port diameter?
Believe it is 0.085ish". Measured it by trying to stick drill bits of various sizes into it. The gas port was drilled by barrel manufacturer, Craddock Precision.

Did you start with the gas fully-open?
Yes, gas full on. It is an Odin Works block. I had same issue w/ a non-adjustable gas block. Bolt appears to 'catch up' on the middle of an empty magazine, suggesting I don't have enough gas in the system. There are no obvious leaks, my gas rings are staggered, and the gas key is very secure.

For a +2” ELGS, I don’t think I would mess around with choking the gas there because the port pressure will already be much lower than RLGS.

@LRRPF52 - replies above in bold.
@hlee - I will get one of those JP Silent Springs if that will solve problem.

THANKS FOR HELP !

 

[357Max]

@LRRPF52 - replies above in bold.
@hlee - I will get one of those JP Silent Springs if that will solve problem.

THANKS FOR HELP !

JP silent capture wont solve anything that a std buffer spring combo wouldn't.
What barrel length for your +2 gas? Dwell time/distance from port to muzzle affects required port size.

Do you have a std wt bolt carrier?

For below example; I've already confirmed gas block alignment, gas tube engagement in key is correct etc. In other words it's an example of tuning a system that does not have other issues.

I have a 224 Valk 18" with a rifle +1" gas length. That's leaves about 3-3/4" - 4" dwell & 224 zips by pretty quick. I started with the std Vltor A5 buffer/spring system, a std weight BCG, & an SLR adj gas block. Shooting 88grn eldm it would fail to feed 1 of 3 with gas wide open & not reliable lock back. Switched from 11.2 oz carrier to 9.5oz and it runs great with some head room in the gas block for 75grn bullets.
Same thing could be accomplished with a lighter buffer, but I just happened to have the lighter carrier in the drawer. Had that not resolved it I'd have opened the gas port a little (.002-.004).

 

[Nivium]

OP started out with the statement that his goal is maximum recoil reduction, but there must be something else driving the primary intent of the rifle. What is that intent? Competition in action-based shooting sports? Hunting? SHTF?

I generally try to build for all three, that is, competition, hunting and SD, with favor to one of them. For this gun, the goal is to get two rounds in the air before the first round hits a hog. I can do this easily with my low mass system in a 5.56, but I wanted a bit more range. I think this is possible with the grendel, but for the second shot to be accurate, I really need to reduce muzzle rise and recoil to the maximum extent possible. I also hope to run this (or at least try it out) as my comp gun. SHTF is the last concern, as my 300BOs are my real SD guns, but reliability is still great to have for a hunting rifle.

 

[jbailey]

I generally try to build for all three, that is, competition, hunting and SD, with favor to one of them. For this gun, the goal is to get two rounds in the air before the first round hits a hog. I can do this easily with my low mass system in a 5.56, but I wanted a bit more range. I think this is possible with the grendel, but for the second shot to be accurate, I really need to reduce muzzle rise and recoil to the maximum extent possible. I also hope to run this (or at least try it out) as my comp gun. SHTF is the last concern, as my 300BOs are my real SD guns, but reliability is still great to have for a hunting rifle.

You should take a look at the 6ARC as a way to reduce recoil. Same cartridge as the 6.5G, just in 6mm. Against those hogs, a 90gr or 95gr bullet, at 2800fps ish, is going to have a very nice recoil profile, especially after you add a nice brake and the other tuning you are planning. Then a 105gr at 2600fps ish is going to outperform the Grendel in the competitions.

Here are the recoil stats from JBM:

6.5 Grendel Recoil
Input Data
Charge Weight:	30.0 gr	Muzzle Velocity:	2400.0 ft/s
Firearm Weight:	7.0 lb	Bullet Weight:	123.0 gr
Output Data
Recoil Velocity:	8.9 ft/s	Recoil Energy:	8.6 ft•lbs
Recoil Impulse:	1.9 lb•s

6 ARC Recoil
Input Data
Charge Weight:	30.0 gr	Muzzle Velocity:	2800.0 ft/s
Firearm Weight:	7.0 lb	Bullet Weight:	90.0 gr
Output Data
Recoil Velocity:	8.0 ft/s	Recoil Energy:	7.0 ft•lbs
Recoil Impulse:	1.7 lb•s

 

[jbailey]

What barrel length for your +2 gas? Dwell time/distance from port to muzzle affects required port size.

Do you have a std wt bolt carrier?

My barrel length is 20" the +2 gas. I have a std wt bolt carrier.
I have tried a Tacom ultra light weight buffer in attempt to lighten up the mass that the gas has to move w/o success.
Maybe I need to drive gas hole out, but don't want to do that before everything else has been tried.

 

[Blutroop]

Start with gas and go from there. If you want to spend more than what’s typical for a buffer, get a JP SCS standard or H2 and a couple weights. Armaspec also makes a competitive alternative but I do not believe it is tunable. Haven’t really looked into that.

I have the armaspec version of silent capture spring (SCS). I’m using it with a low mass fail zero nickel boron bolt (and chamber adaptor) on a 26” 17rem barrel with a plus 2 gas system (maybe +3?) and a adjustable block. I drilled my hole with caveman tools and have a small alignment issue with a .90? Hole for gas. I had to enlarge the hole from .65 to get it to cycle.

 

[charnicus]

That’s wild. Personally have not had to enlarge a gas port.

 

[357Max]

That’s wild. Personally have not had to enlarge a gas port.

If the barrel manufacturer did due diligence you shouldn't have to. Growing pains happen with new calibers especially when combined with ++ gas length.
As the gas length grows so must the port. If extended gas also decreases dwell then the port has to grow even more.

Example:
I had a 18" mid gas barrel on a DPMS 308 with a .074 gas port & SLR adj block that required 8 clicks open (about half way)
I switched the barrel to 18" rifle gas and it had an .090 port & again SLR block tuned at 8 clicks open.
Dwell time decreased since barrel length stayed fixed so port had to be bigger.

You need to be cautious if considering drilling your port.

Pop quiz:

In the above example. Did the port increase by 22% or 47%???

 

[charnicus]

If the barrel manufacturer did due diligence you shouldn't have to. Growing pains happen with new calibers especially when combined with ++ gas length.
As the gas length grows so must the port. If extended gas also decreases dwell then the port has to grow even more.

Example:
I had a 18" mid gas barrel on a DPMS 308 with a .074 gas port & SLR adj block that required 8 clicks open (about half way)
I switched the barrel to 18" rifle gas and it had an .090 port & again SLR block tuned at 8 clicks open.
Dwell time decreased since barrel length stayed fixed so port had to be bigger.

You need to be cautious if considering drilling your port.

Pop quiz:

In the above example. Did the port increase by 22% or 47%???

Personally I wouldn’t drill out my own gas port. I’d investigate other things first and maybe run with more standard components first. I rolled around with a 24” 6.5 Creedmoor before deciding that it was leaving a more practical gun on the table. In that I ran both a JP LMOS and FMOS. Cycled well with either regardless of buffer weight. Just a tweak of the gas screw but was never on the verge of being out of bounds. A plus 3 gas length sounds like too much. Then again I could probably learn a lot more about the AR operating system. I kinda wish I had had HCS out an adjustable block on my Mod1 upper. Some non Black Hills ammo may change that opinion.

My maths say 22%.

 

[357Max]

I wouldn't normally drill out the port either unless it didn't seem right i.e. they drilled the same port in a +2" rgs as they do in a std rgs.

In the example I posted it was a proof 18" rifle gas barrel at .090. They got it right & in the 308 there is enough data that I was able to determine it should measure .089 - .092 before I'd assembled it. In a 6 ARC + 2" rgs I doubt there is much good data out there yet.

My maths say 22%.

Nope it is a 47% increase. For gas flow it's not % size increase that matters (+22%). It's the flow area increase (+47%) that counts.
Small changes in hole size make a big difference in Flow area.

.074 pie x r sq = .017 flow area.
.090 pie x r sq = .025 flow area.
/\ =22% /\ = 47%

 

[jbailey]

In the above example. Did the port increase by 22% or 47%???

You increased the area by 47%.
Area of circle = pie * r^2.
in this case, 0.073 diameter has radius of 0.037 and 0.09 diameter has radius of 0.045. Squaring both radii and multiplying each by 3.14 yields an area of 0.0043 sq inches and 0.0064 sq inches respectively, a 47% increase.

 

[charnicus]

I wouldn't normally drill out the port either unless it didn't seem right i.e. they drilled the same port in a +2" rgs as they do in a std rgs.

In the example I posted it was a proof 18" rifle gas barrel at .090. They got it right & in the 308 there is enough data that I was able to determine it should measure .089 - .092 before I'd assembled it. In a 6 ARC + 2" rgs I doubt there is much good data out there yet.



Nope it is a 47% increase. For gas flow it's not % size increase that matters (+22%). It's the flow area increase (+47%) that counts.
Small changes in hole size make a big difference in Flow area.

.074 pie x r sq = .017 flow area.
.090 pie x r sq = .025 flow area.
/\ =22% /\ = 47%

I did two renditions with Proof .308 barrels. Not only do they hammer but they run well.

Also sounds like a need to get back into school books haha. But good stuff!

 

[jbailey]

In a 6 ARC + 2" rgs I doubt there is much good data out there yet.

So you are saying that there isn't enough info out in the market yet to understand if a 0.0825"ish port provides enough gas in +2" 6ARC with a 20" barrel...
Certainly there are guys successfully running 6ARC at 20" w/ +2's but we don't know what port size they have.

I can't go any lighter on the buffer (carbine Tacom is damn light), I have a -10% spring in there. I could certainly drop weight on the BCG but light weight BCGs are out of stock everywhere and that is an expensive route to go w/o knowing if it solves the problem.

I guess I could try to run some factory loads... maybe N540 isn't a good powder for cycling semi autos (kind of doubt that however).

 

[357Max]

So you are saying that there isn't enough info out in the market yet to understand if a 0.0825"ish port provides enough gas in +2" 6ARC with a 20" barrel...
Certainly there are guys successfully running 6ARC at 20" w/ +2's but we don't know what port size they have.

I can't go any lighter on the buffer (carbine Tacom is damn light), I have a -10% spring in there. I could certainly drop weight on the BCG but light weight BCGs are out of stock everywhere and that is an expensive route to go w/o knowing if it solves the problem.

I guess I could try to run some factory loads... maybe N540 isn't a good powder for cycling semi autos (kind of doubt that however).

That is precisely the problem. While there may be 20" 6 arc with +2" running well; who built them? If a builder goes to the trouble of figuring it out on something new it gives them an advantage i.e. They sell more builds. There is no incentive to put that info out quickly. A barrel maker may just not know & guess at it.

I try to approach trouble shooting logically. If the gun won't run with a proven BCG/buffer spring combo, then I look elsewhere. I do that in a particular order of most common problems first. If all mechanical problems are ruled out such as miss aligned gas block, gas tube engagement in carrier, gas key not blocked or leaking, & good ring seal. Then all arrows start pointing at port size.

If the items I described above have been checked & based on you having a 1 oz buffer with a -10% spring it sounds like a small port to me.

You could go on quick load and try to figure out how much pressure drop off your getting @ that +2 length, but again I don't know of a generic port size -pressure formula that works. There me be one?

I'd suggest searching for plus 2" gas port size on 68 or Grendal forums. 6 x 6.8 and 6 Grendal are close enough that you may find a good port size range to get you close.

@Constructor could probably tell you if .0825 is small for your set up? He's built a metric shit ton of 6mm wildcats, not sure how many where + on gas though.

 

[Constructor]

That is precisely the problem. While there may be 20" 6 arc with +2" running well; who built them? If a builder goes to the trouble of figuring it out on something new it gives them an advantage i.e. They sell more builds. There is no incentive to put that info out quickly. A barrel maker may just not know & guess at it.

I try to approach trouble shooting logically. If the gun won't run with a proven BCG/buffer spring combo, then I look elsewhere. I do that in a particular order of most common problems first. If all mechanical problems are ruled out such as miss aligned gas block, gas tube engagement in carrier, gas key not blocked or leaking, & good ring seal. Then all arrows start pointing at port size.

If the items I described above have been checked & based on you having a 1 oz buffer with a -10% spring it sounds like a small port to me.

You could go on quick load and try to figure out how much pressure drop off your getting @ that +2 length, but again I don't know of a generic port size -pressure formula that works. There me be one?

I'd suggest searching for plus 2" gas port size on 68 or Grendal forums. 6 x 6.8 and 6 Grendal are close enough that you may find a good port size range to get you close.

@Constructor could probably tell you if .0825 is small for your set up? He's built a metric shit ton of 6mm wildcats, not sure how many where + on gas though.

A normal 5.56 20" barrel with a rifle gas needs a .093 port so no a .082 is not large enough. There is a formula that is a half page long but the short way is this- because the case has more volume of burning powder you could figure .002 less if it was a rifle gas but since it is a plus 2 you need to increase the port size. Since I have made a few hundred 6mm Grendels with a +2 gas I can say the port needs to be .096-.098. That depends on buffer weight, spring pressure, if there are no gas leaks, gas tube in carrier key fit is correct and bolt tail in carrier fit is correct along with pressure generated by the cartridge.

 

[ormandj]

I've got a Craddock 18" rifle-length + 1" 6mm ARC, though I do run an AGB. I'm using the ADM heavy buffer setup (think of it like a H2) with normal spring and a full weight BCG, and have perfect functioning with/without suppressor. If he suggested you go with the +2", I'd assume he had tested it.

I didn't measure my gas port, but I did shoot an email to Paul to get an idea what he's using, as I don't want to tear down my rifle to use pin gauges to figure it out right now. I'll let you know whenever I hear back. I don't know if it'd be the same as yours, since you have a +2, though.

Have you tried reaching out to the shop? They're super nice over there, and I'm sure they'll take care of you. I wouldn't be happy running reduced weight carriers and jumping through all the hoops you have to try and get your rifle to function. Something sounds very off there to me.

 

[357Max]

@jbailey - I'd take Constructors port recommendation to the bank. Drill it .096, put a regular buffer/spring back in there & just tune the gas block.

Hell I see your in Arlington. If you want to pull the barrel and run it up to Annapolis area I'd be happy to put it in the mill & drill it for you with a #41 drill = .096. Free of coarse.

PM me if you want to do that.

 

[Yondering]

So you are saying that there isn't enough info out in the market yet to understand if a 0.0825"ish port provides enough gas in +2" 6ARC with a 20" barrel...
Certainly there are guys successfully running 6ARC at 20" w/ +2's but we don't know what port size they have.

I can't go any lighter on the buffer (carbine Tacom is damn light), I have a -10% spring in there. I could certainly drop weight on the BCG but light weight BCGs are out of stock everywhere and that is an expensive route to go w/o knowing if it solves the problem.

I guess I could try to run some factory loads... maybe N540 isn't a good powder for cycling semi autos (kind of doubt that however).

Before you drill anything - if you think the rifle is undergassed (which is pretty likely with a +2" on a 20" barrel IMO), try a full load of Leverevolution with 105gr bullets. It produces more gas than most other powder options for this round and will cycle better than something like RL15 for sure. No idea about N540 though.

Personally I use a 24"and a 19" both with standard rifle gas (243 LBC rather than 6 ARC but close enough to be the same for this discussion) and see zero reason to use a +2" system on the 24", and definitely wouldn't on a shorter barrel. Not that it can't work, I just don't see any reason to make the gas system that long on this. You definitely can go too long on the gas system and end up with cycling problems and a very limited selection of powders that work.

My suggestion though, since you've already got the barrel - go back to standard weight spring and buffer, and get the gas figured out. Then if you're still having feed issues, it's probably a magazine thing. Playing with a bunch of non-standard parts when you're having feed issues and don't understand the cause is a solid recipe for frustration. Getting the gas right is priority #1.

 

[jbailey]

Thanks Everyone that had ideas. I learned a lot and was reminded on why Hide is such a great resource. So many knowledgeable folks willing to help a guy out.

I get to play with firearms once or twice a month, so I will try some of these ideas next time out. @357Max will drop you a PM per your offer.

 

[Nivium]

I have a light spring, buffer, and carrier setup for a subsonic-only 300 Blk; I don't have spring force numbers for you but have ended up with a spring several inches shorter than standard and it still strips rounds without issues. There is quite a bit of room to play and tune the AR15 system in every aspect, just change one thing at a time and try to understand what each change does for you.

Can I ask the benefit of a light buffer/carrier setup with the subsonic-only 300 BLK? I run my subsonic blk with H2 buffer and heavier carrier, as that is what it seems like people recommend. That said, it has quite a bit of kick (compared to my unsuppressed 300 blk with lighter bcg/buffer) and I would love to run a low mass system suppressed if it is feasible

 

[Yondering]

Can I ask the benefit of a light buffer/carrier setup with the subsonic-only 300 BLK? I run my subsonic blk with H2 buffer and heavier carrier, as that is what it seems like people recommend. That said, it has quite a bit of kick (compared to my unsuppressed 300 blk with lighter bcg/buffer) and I would love to run a low mass system suppressed if it is feasible

The benefit is twofold:

- mainly less noise; specifically the ability to use a faster burning powder for suppressed subsonic loads and still have the action cycle. The use of faster powders can result in a quieter suppressed shot by producing less gas; this is realized both at the muzzle and at the ejection port.
- also, lower reciprocating mass = less movement when the bolt slams home. Rapid fire can be pretty stable with this setup, using a load that matches it.

It can also let the action cycle with lighter bullet weights for subsonic loads. Mine will cycle 150gr subsonics with Lil'Gun when using that spring/buffer combination. Most of the time I just use a standard spring & carbine buffer though, combined with the extra light carrier, and have good reliability with both supers and "normal" 200+gr subsonics.

 

[supercorndogs]

This thread should be called reciprocating weight. The AR-15 operating system is the meat bag running it. Mine is high mass.

 

[Nivium]

The benefit is twofold:

- mainly less noise; specifically the ability to use a faster burning powder for suppressed subsonic loads and still have the action cycle. The use of faster powders can result in a quieter suppressed shot by producing less gas; this is realized both at the muzzle and at the ejection port.
- also, lower reciprocating mass = less movement when the bolt slams home. Rapid fire can be pretty stable with this setup, using a load that matches it.

It can also let the action cycle with lighter bullet weights for subsonic loads. Mine will cycle 150gr subsonics with Lil'Gun when using that spring/buffer combination. Most of the time I just use a standard spring & carbine buffer though, combined with the extra light carrier, and have good reliability with both supers and "normal" 200+gr subsonics.

Are you handloading these fast-powdered subs? Do you have any issues with the suppressor fouling up and causing the lower mass system to fail?

 

[Greg Langelius *]

Having both bought and built 6.5 Grendel AR-15's, I've found that the basic AR-15 buffer handles cycling issues fine, that the recoil of the Grendel is not really all that objectionable, and I happen to be very recoil sensitive. I simply slipped a Limbsaver Pad over the butt and recoil became absolutely negligible.

I did put a brake on, a Linear Compensator. This device tends to have negligible effect of recoil; its primary advantage is to redirect sound energy out to the front, where it reduces or eliminates lateral shock waves and reduces disturbance to other shooters, the opposite of what a conventional brake would do.

With all the slick bells and whistles available these days, we are presented with an overload of options; and this inevitably leads to overthinking. Those new to the process tend to grasp at every advantage, whether needed or not, in an effort to build the very best implement available. The fact is, the basic system works quite well; and when we get to messing with it, we find as many problems as solutions.

One other quite important point. When handloading the 6.5 Grendel, take extra care to resist the temptation to load above max published loads. The velocities appear slower and the temptation is great. But the larger base diameter adaptations to the standard AR bolt face tend to weaken the extractor, and charges above max will snap it in half, as was my own experience. The case pressure indicators look acceptable, but the pressures are enough to occasionally snap extractors.

JP makes an improved Grendel extractor that addresses this problem. I installed one as a replacement for the broken one; but now, I also stay within published load limits, too.

Greg

 

[r.tenorio671]

...most AR15 format barrels are ported to meet the operational requirements of the standard M4, i.e., standard M4/M16 BCG and a H buffer & carbine recoil spring. If I recall correctly, even the caliber variants like the Grendel are based on the M4's operating pressures of 52K-62K PSI for .223 REM/5.56NATO ammo. the Grendel SAAMI spec for pressure is 52K PSI

 

[Yondering]

Are you handloading these fast-powdered subs? Do you have any issues with the suppressor fouling up and causing the lower mass system to fail?

Yes, handloading of course. You can't buy anything like that.

No issues at all. I'm not sure what you mean about the suppressor fouling up and causing the low mass system to fail though; those things aren't really related. All suppressors collect fouling, but that has very little to do with the rifle's operation. If you're asking about partially burnt powder collecting in the suppressor - no, that's part of the reason for using a faster burning powder (to keep peak pressure high enough for a clean burn) instead of just downloading a slower powder like 1680.

If you're asking about fouling building up in the receiver - no I haven't had any issues caused by that. I keep my ARs well lubed though* and that keeps them running well even when they're pretty dirty. And mine do get very dirty. I try to clean them once a year whether they need it or not.

*exceptions being very low temps or very dusty environments; I clean and then minimally lube the internals for those environments but that's pretty rare where I live.

 

[Nivium]

Yes, handloading of course. You can't buy anything like that.

No issues at all. I'm not sure what you mean about the suppressor fouling up and causing the low mass system to fail though; those things aren't really related. All suppressors collect fouling, but that has very little to do with the rifle's operation. If you're asking about partially burnt powder collecting in the suppressor - no, that's part of the reason for using a faster burning powder (to keep peak pressure high enough for a clean burn) instead of just downloading a slower powder like 1680.

If you're asking about fouling building up in the receiver - no I haven't had any issues caused by that. I keep my ARs well lubed though* and that keeps them running well even when they're pretty dirty. And mine do get very dirty. I try to clean them once a year whether they need it or not.

*exceptions being very low temps or very dusty environments; I clean and then minimally lube the internals for those environments but that's pretty rare where I live.

Yeah, I meant fouling up the upper. Thanks for the response

 

[3GN_Lurker]

I have a JP silent captured spring behind a JP stainless low mass bcg. While its in 556 it has been completely reliable and soo damned smooth compared to a milspec setup.

 

[TonyTheTiger]

@Nivium how did this end up for you? I'm a huge proponent of tuning for low recoil, but for my 6.5 and 6 Grendels I typically go heavy just because I'm more likely to push hotter loads in in them than a 5.56. I've shot a factory JP 6.5 Grendel with a low mass system quite a bit, and it works great and gun movement is minimal even though it's fairly light, but I can load a little hotter in my full mass systems. And even the the recoil is what I'd call light, it just feels a little more jarring than a similarly tuned 5.56.
And yes, the Similar Threads function brought me here.

 

[Yondering]

Interesting. I have not observed that phenomenon with loading hotter in heavy mass carriers with my guns. The light carriers seem to handle the same hot loads just fine, they are just a little more picky about being tuned right for the load. While a heavy carrier & buffer may work well with both a mild load and a hot load (although we're really talking more or less gas, not peak pressure), a light weight carrier setup that's tuned for the mild load is much more likely to be overgassed with the hot load. Once they're tuned right though, both seem to be equally capable of handling hot loads, and the cartridge or caliber doesn't matter.

 

[TonyTheTiger]

Interesting. I have not observed that phenomenon with loading hotter in heavy mass carriers with my guns. The light carriers seem to handle the same hot loads just fine, they are just a little more picky about being tuned right for the load. While a heavy carrier & buffer may work well with both a mild load and a hot load (although we're really talking more or less gas, not peak pressure), a light weight carrier setup that's tuned for the mild load is much more likely to be overgassed with the hot load. Once they're tuned right though, both seem to be equally capable of handling hot loads, and the cartridge or caliber doesn't matter.

I agree. You went further in depth than I did. I should've included that I set a gas block to run with a light load then never re adjust for a heavy load. I've found full mass setups more tolerant of this than a low mass setup. A way overgassed heavy reciprocating mass is just harsher than necessary. An exceedingly overgassed low mass system seems likely to rip case rims in my experience.
Like you said, tuning fixes all this but if you're not going to do that a heavy system is more forgiving.