Help on pure precision 6.5 Grendel

[Nivium]

I'm hoping for some help from the Hide on finishing a pure precision ar grendel for a split hunting and competition build. I understand the instinct to give "the difference is negligible" and "it doesn't matter" responses, and generally I appreciate them. However, the fun of building an AR for me is truly tuning everything to be as absolutely accurate as possible for whatever purpose it is that I built the gun. For this gun, I am shooting for absolute maximum precision out to at least 700. So, this thread is for the min/max'ers out there who understand the obsession with mechanical perfection notwithstanding cost or otherwise external limitations.

At this point in the build, my question relates to ideal handguard and upper receiver mating. My 6.5 CM ar10 was built somewhat early in my progression through the hobby, so I just threw a Bartlein on a thermofit upper (BCM I think). It was a tack driver (1/3-.5 moa gun), but on loading a bipod there was a slight POI shift and even some spreading of the groups. I swapped out the BCM upper and put it on a monolothic billet upper/handguard system. The barrel fit wasn't as snug, but I still lapped and bedded it with shims, etc. The POI issue disappeared completely, but the groups spread out slightly and I attribute this to the fact that the barrel wasn't in a super tight upper (though it was shimmed to be very tight).

Now I am trying to build the perfect 6.5 Grendel, and torn as to whether there is a larger benefit to the thermofit upper or the increased rigidity of an upper that connects directly to the handguard (Seekins IRMT-R, Larue Stealth 2, Aero enhanced, monolithic uppers to the extent they are still in production). Has anyone experimented with the two enough to build an opinion on which is better? Better yet, does anyone know of a manufacturer that has both very snug, preferably thermal-fit uppers with a compatible rail that connects directly to the upper as to create a truly free-float rail system that does not touch the barrel nut?

It appears based on the options, I am forced to decide between very tight uppers and a more monolithic rail system, but I would love to be corrected if I am wrong. I also know that the geiselle rails are said to be great, though they aren't truly free float. Does anyone have an opinion as to whether the added rigidity of geiselle's proprietary barrel nut outweighs the potential rigidity benefits of a connected upper/handguard set?

And finally, another accuracy element I have not attempted to reduce before is the negative impact on accuracy that results from play between the BCG and the upper. For this build I am going with a JP LMOS bcg and enhanced bolt, so I am hoping this isn't an issue. However, I know that Wilson billet uppers have tighter tolerances inside the upper to remove the play of the BCG. The wilson upper, however, is not monolothic and has no options for direct attachment to the rails.

Current build thus far:

20" Proof SS straight-taper barrel (will be shimmed for the build) with rifle length gas
JP LMOS bcg and enhanced bolt
Master of Arms adjustible gas block
Battle arms lower
Triggertech diamond trigger
A2 stock with either Taccom lightweight buffer system or JP SCS
all the other stuff that doesn't matter. Just can't decide on the upper, handguard and barrel nut

Advice would be hugely helpful. Apologies for the long-winded question, and thanks in advance

 

[bigjake83]

@ccasanova

 

[ccasanova]

Larue Stealth 2.0 Upper/Rail + JP BCG + JP SCS is a solid, smooth shooting setup. @bigjake83 built an upper for me using those. It hammers at distance, and is more than robust. I’ve had the Seekins, which is also nice, but prefer the LaRue.

 

[Nivium]

Thanks for the response. Do you recall if the barrel was a snug fit in the larue?

 

[ccasanova]

Thanks for the response. Do you recall if the barrel was a snug fit in the larue?

@bigjake83

 

[bigjake83]

Thanks for the response. Do you recall if the barrel was a snug fit in the larue?

No it's not a Thermo Fit. Use some Red 271 or 272 Loctite, apply an even coat around outer area of barrel extension and inside receiver where the barrel slides into. Insert barrel and immediately wipe off all excess by receiver threads, apply some Anti-seize on receiver threads then torque barrel nut between 50-60 Ft Lbs. After the barrel nut is torqued clean up all excess around feed ramps and inside of receiver. If you want a more permanent fit use Green 620 as directed above.

 

[Nivium]

Awesome, much appreciated. Any idea what is the most rigid conventional setup (not monolithic or connecting directly to the upper)? There seems to be some debate as to whether JP or Geiselle has the best barrel nut, and I'm not aware of any other manufacturers that make really strong barrel nuts meant to distribute potential pressure on the handguard.

 

[bigjake83]

Awesome, much appreciated. Any idea what is the most rigid conventional setup (not monolithic or connecting directly to the upper)? There seems to be some debate as to whether JP or Geiselle has the best barrel nut, and I'm not aware of any other manufacturers that make really strong barrel nuts meant to distribute potential pressure on the handguard.

JP would probably be the most rigid, but the Geissele is definitely a close second, And since the Geissele utilizes common m-lok attachments and has better style that would be my vote.

 

[Nivium]

Anyone else have insight on this? Particularly, whether thermofitting will afford more precision benefits than a monolothic upper, or vice versa?

 

[Strykervet]

I bedded a Satern bbl. to an MUR1 receiver I faced first. Used Loctite 620 (permanent green). URX4 handguard. It's a one piece unit and if you bed the barrel this way it forms what's essentially a one piece upper. Now when the barrel is toast so is the upper but it's a minimal cost for the gain if positive surety is what you're after. Nothing will move.

I did a Krieger barreled 5.56 16" the same way. Same results.

You could even double down and loctite the handguard too but I've never seen one of those work loose before and it wouldn't affect the barrel mating if it did. Another option would be to put a clamp on the rail between the upper and handguard. But again, don't see a reason and that's a $279 part you can salvage along with the maybe the gas block and suppressor mount.

I don't know why I did all that because it already shot .33MOA to begin with with 120gr. Nosler BT handloads (but good for maybe 1MOA with various others, it's a one trick pony I guess but it's a good one!). I figure it's "mo betta" now? Mo solid for sure, I know nothing is gonna move. Ever.

A JP FMOS BCG rounds it all out, where the cut rifle Satern bbl. lends precision the JP FMOS lends reliability. Not to mention many barrels can be had fitted with a JP bolt now.

I face all the receivers now that I have that tool but I only bed the best ones.

 

[LRRPF52]

I’ve never noticed a bipod loading issue shooting Grendel, and I have a bunch of them. I don’t really load them up like that though, since there isn’t much recoil.

I’ve been fitting barrels to squared uppers, bedding, press-fitting, thermo-fitting, whatever works for the particular barrel extension-to-upper fit. Been using a lot of Lilja barrels.

I have the complete LaRue 18” Stealth 2.0 rifle he sent me. Even with a 1.5-6x42 MTAC, I was able to shoot a 5rd group into .545” with bulk Hornady American Gunner. I think that barrel is just dropped-in from the factory. The handguard is totally isolated from the upper.

I don’t know that I would mess with the JP LMOS. Better to go with a Full Mass BCG, but it might work with RLGS 20” and proper recoil system set-up. I currently use Bootleg Adjustable Gas carriers since they’re so easy to adjust. Unless I see any failures or weird accumulation in the regulator, I think they’ll be my go-to carriers from hereon out.