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Overthinking it? Upper receiver selection choice for accurized SPR build (not a match rifle)

Amatsukaze_DD

Amatsukaze_DD

Weeaboo Prime
Minuteman
Jun 7, 2022
67
44
Fletcher, NC
Recently picked up the SiCo lower on the brownells sale, and managed to snag a criterion 18" immediately after. So my spr build is happening sooner rather than later. I am wanting to know if, for an 18" small frame, going with an inverse barrel nut system like a seekins or m4e1 enhanced gives any real, tangible benefits to what is meant to be an accurate duty/innawoods rifle.

I am *NOT* trying to build out a sub moa match rifle. If i had wanted to do that I would have been looking at a CLE/WOA/Bart or whatever barrel. I went with the criterion because it's chrome lined and should be at least moa. Anything past moa is just a plus.

What I *AM* trying to do is get a roughly 1 moa rifle that I can abuse the piss out of and brace against all number of barricades or bipods and it not effect the POI. The rifle will be shot primarily suppressed, and is going to be sporting a lightweight mpvo for shooting out to around 700.

I do have experience with the seekins upper on my sp10, and while I think that handguard is a fantastic match/dmr style rifle, I'm not convinced it would be comfortable to shoot off shoulder, which this rifle will be done a lot. I know Aero's m4e1 enhanced uses the same built in barrel nut system as the seekins. Any other companies that do this? Are there any real, tangible benefits to using this system for a small frame compared to a nice thermal fit upper like a mega?
 
I like the Seekins/Mega/Aero systems, but none of the handgaurds are great to hold onto for offhand shooting. I think their advantage is more apparent with heavy guns and heavy barrels. For your uses and on a gun that will be carried enough that weight and ergos are important I would absolutely prefer a simple mil spec type receiver with a tight bore, like a BCM or B. Kings upper.
 
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I suggest taking a micrometer to your extension to check for taper. I've run across a couple that have close to 0.001" taper (larger at the flange end). This gives a slip fit until the last 1/4 to 3/8". The extension can be loose except for the very end of the nose. How the threads are cut/rolled can also make the nose tighter than the bore. Feels like a thermal fit but ain't. Hopefully the nose has been squared AFTER the threads are made.

A heavy walled upper gives better support (at a weight penalty).
Look at where the extension goes. Where do you want a good fit?
Axial support is compression of the flange to the threaded nose.
Lateral support is in the receiver body.
AR-Barrel-Support.jpg


I also do a spray on shim and burnish it down to 0.002" over (0.001" per side).
This REQUIRES a hot receiver/cold barrel to slip together.
Spray-On-Shim.jpg
 
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TheOE800 said:
No, not like a BCM or Zev. You’d want to bed with 620.
Click to expand...
Looking into the zev MML lineup, it looks like these would be both a thermal fit upper and have the barrel nut machined into the upper like the m4e1E and a seekins, but has a more traditional style of hg to hold on to. Anyone have experience with the MML? I should add that I have pretty small hands and prefer thinner style HG's, I normally go with SLR rails just for comforts sake. I don't hate a girthier handguard, its still plenty usable for me.
 
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Amatsukaze_DD said:
Looking into the zev MML lineup, it looks like these would be both a thermal fit upper and have the barrel nut machined into the upper like the m4e1E and a seekins, but has a more traditional style of hg to hold on to. Anyone have experience with the MML? I should add that I have pretty small hands and prefer thinner style HG's, I normally go with SLR rails just for comforts sake. I don't hate a girthier handguard, its still plenty usable for me.
Click to expand...
The Mega is definitely girthier than anything like an SLR. Not as bad as the Aero M4e1 or the JP round handguards (just remembered JP has a semi monolithic system too) and way more hold-able than the Seekins.
 
For what you're doing & the barrel you're using, you surely don't need a monolithic upper.

Vltor MUR, BCM, Cross Machine, SanTan are all some of my favorites; BCM is a thermal fit & SanTan usually is to most barrel extensions.

BCM's are very concentric & blems are usually available for around $60...............haven't checked lately; out of a dozen or so that I've used, I've never been able to find a blem on them.

If you don't want to use epoxy / glue on a non-thermal fit combination, High Temp engine paint applied to the barrel extension & sanded to fit works well..........................if you really think your build justifies it.

MM
 
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Guess I have never built anything with the thought of “ABUSING THE PISS OUT OF IT”

Lot of my gear has seen hard use, lot of wear and some inadvertent damage but never intentional abuse.
 
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Amatsukaze_DD said:
Looking into the zev MML lineup, it looks like these would be both a thermal fit upper and have the barrel nut machined into the upper like the m4e1E and a seekins, but has a more traditional style of hg to hold on to. Anyone have experience with the MML? I should add that I have pretty small hands and prefer thinner style HG's, I normally go with SLR rails just for comforts sake. I don't hate a girthier handguard, its still plenty usable for me.
Click to expand...
The MML is what I use for dedicated precision rigs. The receiver bore is tight and I have seated barrels thermally, but I’ve also seen examples where the barrel could be mated without heat - not that it was loose, but you didn’t need to expand the bore. The handguard is fairly large in hand IMO and adding any kind of cover or grip surface to hold onto instead of slippery anodizing only makes that worse if you’re on the medium to small side.
 
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TheOE800 said:
They are no longer thermal fit. I see they offer the option, but I don’t think they get them produced anymore.
Click to expand...
Huh... Good to know.

Must be a recent thing? I was looking at their website the other day and thermal fit did not indicate "out of stock" (but as of this post, it does). Which makes me think they may have had some in recently.
 
Amatsukaze_DD said:
Seems like bcm and zevs are the only readily accessible thermal fit uppers. I'd rather thermal fit than bed the rifle so one of those it is
Click to expand...
SOLGW uppers are also thermal fit. They also have a tighter fit on all the various brand lowers I have tried them on. IMHO, a tighter fit there helps with more consistent wielding of the firearm. ( Shouldering )
FWIW, BCM Lower was the tightest.

However it is just a standard Forged lower... IE not strengthed at the barrel nut area.
 
Naaman said:
Huh... Good to know.

Must be a recent thing? I was looking at their website the other day and thermal fit did not indicate "out of stock" (but as of this post, it does). Which makes me think they may have had some in recently.
Click to expand...
Maybe we’re both right. They stopped producing them as thermal fit mid-pandemic, perhaps there was a clamor to bring them back.
 
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Naaman said:
That would be nice.
Click to expand...
I swear my buddy just ordered a centurion upper and it was thermal fit. Maybe he had it laying around but I doubt it.

Talked to zev cs, the MML isn't technically meant to be thermal fit, it's just held to within a thou tolerance so it can be if the barrel extension is slightly over. If I'm lucky the criterion will be slightly over but I'm probably not that lucky. I'll probably have to bed it with loctite if I go that route. It's still kinda up in the air if I really truly need a semi monolithic reciever. I have plenty of time to think about it either way.
 
Could get a regular M4 upper receiver and a KAC URX 4. The larue upper is another option I think it's 490 with a 13" handguard secured by four bolts. I didn't have good luck with the aero m4e1/m5e1 but ymmv.
 
BCP said:
Could get a regular M4 upper receiver and a KAC URX 4. The larue upper is another option I think it's 490 with a 13" handguard secured by four bolts. I didn't have good luck with the aero m4e1/m5e1 but ymmv.
Click to expand...
If I go with a standard style upper it'll be either a bcm or a solgw. I'm not sure if an ultra thin hg like an slr ion would be suitable, it's definitely light and fits my smol femboy hands well, just not sure if it would cause poi shift with a lot of force, being that small.
 
Amatsukaze_DD said:
If I go with a standard style upper it'll be either a bcm or a solgw. I'm not sure if an ultra thin hg like an slr ion would be suitable, it's definitely light and fits my smol femboy hands well, just not sure if it would cause poi shift with a lot of force, being that small.
Click to expand...
It will definitely deflect under load.
 
Rocketvapor said:
I suggest taking a micrometer to your extension to check for taper. I've run across a couple that have close to 0.001" taper (larger at the flange end). This gives a slip fit until the last 1/4 to 3/8". The extension can be loose except for the very end of the nose. How the threads are cut/rolled can also make the nose tighter than the bore. Feels like a thermal fit but ain't. Hopefully the nose has been squared AFTER the threads are made.

A heavy walled upper gives better support (at a weight penalty).
Look at where the extension goes. Where do you want a good fit?
Axial support is compression of the flange to the threaded nose.
Lateral support is in the receiver body.
View attachment 8154813

I also do a spray on shim and burnish it down to 0.002" over (0.001" per side).
This REQUIRES a hot receiver/cold barrel to slip together.
View attachment 8154814
Click to expand...

What product are you using for the spay on shim?
 
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To remove and swap barrels, warm the receiver (aluminum expands more than steel) tap it out.
I don't do much to assemble an upper. Most have been heavy wall BCA right side charging lately.
I face the nose with the wheeler tool, spray and cure shim, burnish with 2000 wet paper, Heat receiver in the oven, stand barrel in the freezer.
Slip together and hand tighten a barrel nut. Let stabilize.
P1090361.jpg
 
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TonyTheTiger said:
I like the Seekins/Mega/Aero systems, but none of the handgaurds are great to hold onto for offhand shooting. I think their advantage is more apparent with heavy guns and heavy barrels. For your uses and on a gun that will be carried enough that weight and ergos are important I would absolutely prefer a simple mil spec type receiver with a tight bore, like a BCM or B. Kings upper.
Click to expand...
I like the m4e1 alot. I also REALLY like the aero handguard. Nice and large, nice and round. Feels great IMO.
 
TheOE800 said:
What other large cylindrical things you like in your hands?
Click to expand...
It's not actually perfectly round, it is a little flat at 9,12,3 and 6 oclock. So if you do want to rest it on something it works well.
 
I just assembled two accurate-ized uppers (well, thats as intended, verdict is still out on the outcome...) One was a 308 using a Mega receiver and one of SLR's newer hand guards with the anti rotation pin. This one did not need thermal fitting. Also, I had to use the SLR jig to drill into the Mega. Todd @ SLR was very helpful though I jumped the gun and drilled before realizing the first 1913 lug on the Mega is sort of normal sized. So the pin hole is exposed between the first and second lug. I'm calling it character.

From a fit and finish perspective, the combo looks and feels great.

The second upper used a small frame Mega receiver-rail set, which I really like. The diameter limits what grip panels and such I put on, as it gets a bit big for my hands, but I *much* prefer it to the Aero system, which the Mega was replacing, and which is much too large for me. The fit and finish is really nice, but they are not cheap.

This one, with a V7 barrel, did require a night in the freezer and heat gun, and then some encouragement with a mallet. But the end product is really nice. Its either going to shoot reealy well or... I'll leave it on the wall and say it does.

My feeling is the Mega receiver handguard interface would put up with a lot of use, brace pressure, and such, but I don't have any practical experience to support this.
 
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Deepwoods said:
I just assembled two accurate-ized uppers (well, thats as intended, verdict is still out on the outcome...) One was a 308 using a Mega receiver and one of SLR's newer hand guards with the anti rotation pin. This one did not need thermal fitting. Also, I had to use the SLR jig to drill into the Mega. Todd @ SLR was very helpful though I jumped the gun and drilled before realizing the first 1913 lug on the Mega is sort of normal sized. So the pin hole is exposed between the first and second lug. I'm calling it character.

From a fit and finish perspective, the combo looks and feels great.

The second upper used a small frame Mega receiver-rail set, which I really like. The diameter limits what grip panels and such I put on, as it gets a bit big for my hands, but I *much* prefer it to the Aero system, which the Mega was replacing, and which is much too large for me. The fit and finish is really nice, but they are not cheap.

This one, with a V7 barrel, did require a night in the freezer and heat gun, and then some encouragement with a mallet. But the end product is really nice. Its either going to shoot reealy well or... I'll leave it on the wall and say it does.

My feeling is the Mega receiver handguard interface would put up with a lot of use, brace pressure, and such, but I don't have any practical experience to support this.
Click to expand...
Do let us know how that 2nd one performs, and if you don't mind, could you take a few pictures of the handguard compared to the SLR?

Wild that someone else is doing the exact same thing I'm thinking of doing with the exact same handguards even.
 
This will get me called poor :)
Put together a cheap upper.
$99 BCA side charge (with complete BCG),
$60 Mentium-USA 2 inch diameter handguard 0.936 gas adjustable gas block won't fit under narrow tube.
$350 28" barrel bought as a spare for my 3000 round barrel.
New-Upper.jpg

So, got the upper together and why not just slap together another lower from spare parts?
Schmid 2 stage, light trigger spring, mil disconnector spring.
80% plastic.
H2 buffer and standard carbine spring.
Clearance TI-7 stock.
Topped with a Buras 8-40X50 from Midway on sale, on a 20 MOA Primary Arms mount.
 
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Rocketvapor said:
This will get me called poor :)
Put together a cheap upper.
$99 BCA side charge (with complete BCG),
$60 Mentium-USA 2 inch diameter handguard 0.936 gas adjustable gas block won't fit under narrow tube.
$350 28" barrel bought as a spare for my 3000 round barrel. View attachment 8158096
So, got the upper together and why not just slap together another lower from spare parts?
Schmid 2 stage, light trigger spring, mil disconnector spring.
80% plastic.
H2 buffer and standard carbine spring.
Clearance TI-7 stock.
Topped with a Buras 8-40X50 from Midway on sale, on a 20 MOA Primary Arms mount.
Click to expand...
spare parts blaster is always based. That thing looks like it weighs about a thousand pounds though, a 28" bull barrel and a BCA upper combined is probably the weight of a small car.
 
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Had to shim the Bolt release. Paper hole punch, plastic, drill.
Dremel to the handguard for gas block clearance.
Plastic-shim.jpg

Gas-Block-Clearance.jpg


Still well under F-Open weight. :)
 
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Amatsukaze_DD said:
Seems like bcm and zevs are the only readily accessible thermal fit uppers. I'd rather thermal fit than bed the rifle so one of those it is
Click to expand...

Grey Ghost precision is also Thermal fit. Extremely nice sets with super tight tolerances.


For what you are after Mega mml is very stout solid upper. If you don’t want to use that handguard you could go to there GTR-3h upper that is not stuck to their mono style hand guard but you still have a very robust upper and thermal fit. If you want precision but save weight look at the Grey ghost billet upper. There handguard is also very solid. Pure stout precision mega, for an all around accurate rifle and weight savings I have became a fan of GGP’s billet sets.
 
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B Man said:
Grey Ghost precision is also Thermal fit. Extremely nice sets with super tight tolerances.


For what you are after Mega mml is very stout solid upper. If you don’t want to use that handguard you could go to there GTR-3h upper that is not stuck to their mono style hand guard but you still have a very robust upper and thermal fit. If you want precision but save weight look at the Grey ghost billet upper. There handguard is also very solid. Pure stout precision mega, for an all around accurate rifle and weight savings I have became a fan of GGP’s billet sets.
Click to expand...
its a fine line between precision and weight savings. After scope and suppressor I'm trying to keep the entire rifle under 9 pounds or so, so every bit helps. I think that more or less automatically excludes any integral barrel nut designs like the MML.
 
Amatsukaze_DD said:
I do have experience with the seekins upper on my sp10, and while I think that handguard is a fantastic match/dmr style rifle, I'm not convinced it would be comfortable to shoot off shoulder, which this rifle will be done a lot. I know Aero's m4e1 enhanced uses the same built in barrel nut system as the seekins. Any other companies that do this? Are there any real, tangible benefits to using this system for a small frame compared to a nice thermal fit upper like a mega?
Click to expand...
There are a few other M4E1 enhanced compatible rails mentioned in this thread here:

Aero M4E1 upper receiver? - AR15.COM

Firearm Discussion and Resources from AR-15, AK-47, Handguns and more! Buy, Sell, and Trade your Firearms and Gear.
www.ar15.com www.ar15.com
 
Amatsukaze_DD said:
its a fine line between precision and weight savings. After scope and suppressor I'm trying to keep the entire rifle under 9 pounds or so, so every bit helps. I think that more or less automatically excludes any integral barrel nut designs like the MML.
Click to expand...
Yes there is a considerable weight difference between my Mega mml build and GGP build. The GGP build is still extremely accurate, especially considering I’m the shooter 😅

The aero enhanced sets are not that heavy either. Just not thermal fit. They are not as thick walled as the Mega or Seekins, handguard isn’t as thick either so it allows some weight savings with the ability of a stiffer handguard without inducing stress as easily to the barrel.

This is my GGP set.
4F77864B-2B80-44C2-A499-8F4B3D559EE6.jpeg


986C6829-E9A7-4218-98F5-528EFCF35DE2.jpeg
 
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My precision rigs get weighty, especially scoped and with can. I think my Mega MML based ARC is around 14lbs without a bipod?

B Man said:
Grey Ghost precision is also Thermal fit. Extremely nice sets with super tight tolerances.


For what you are after Mega mml is very stout solid upper. If you don’t want to use that handguard you could go to there GTR-3h upper that is not stuck to their mono style hand guard but you still have a very robust upper and thermal fit. If you want precision but save weight look at the Grey ghost billet upper. There handguard is also very solid. Pure stout precision mega, for an all around accurate rifle and weight savings I have became a fan of GGP’s billet sets.
Click to expand...
I have a spare one of those standard nut billet Megas kicking around in the spare parts drawer somewhere.
 
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TheOE800 said:
My precision rigs get weighty, especially scoped and with can. I think my Mega MML based ARC is around 14lbs without a bipod?


I have a spare one of those standard nut billet Megas kicking around in the spare parts drawer somewhere.
Click to expand...

Sounds like that upper is just really in your way. You need somewhere to put it out your way just let me know 🙃
 
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