Great (interim) success!! I put the Criterion 16” 6.5 Creed barrel in and it feeds just fine, even without letting the bolt slam (though the extractor didn’t grab until it a little bump). I’ve put a Brownells 3-18 MPO on there and done a crude bore-sight. Hopefully it’ll shoot!
What gas system length does it have and what is the port diameter?
What recoil/action spring and RET are you using?
Did you bed the barrel into the upper? I’ve taken Krieger barreled .308 uppers that wouldn’t shoot, bedded them, then started printing .6” 5rd groups with them out of the gate after the re-build. One particular rifle was shooting 1.7” with the same load prior to that. Once I replaced the gas block, gas tube with an ArmaLite tube (it had a standard rifle gas tube on it and was short-stroking), and bedded it, it became a laser.
Especially for a Creedmoor or .260 Rem, I would spring it with a stiff action spring using an A5 RET.
Reliability/Accuracy Formula for a large frame AR-10-ish gasser in 6.5CM or .260 Rem:
1. Start out with the understanding you will only be running a certain projectile weight range reliably. For example, I wouldn’t expect to be able to shoot 107gr and 147gr without tuning to each. 123gr-130gr is a better projectile weight range to settle into for the 6.5-08 type cases in a gasser. 140-147gr makes it harder to get it reliable and not trash your brass, while enjoying the benefits of that weight class at the speeds they need. 123gr-130gr are much easier to get fast and still have some brass life. I’ve seen exceptions, but this is generally what you will see.
2. Only buy quality parts for the critical core pressure-containment (barrel/bolt), operating system, gas system, fire control, LPK, recoil spring and buffer. Dudes focus so much on furniture and coating while running $200 barrels with mass-produced chambers, outsourced BCGs with small parts failing, and band-aids trying to address early issues that pop up out of the gate. The more parts that come from the same reputable manufacturer, the better, especially pressure containment, gas system, BCG, recoil/buffer, and receivers.
3. The components need to be compatible from a mechanical engineering and functioning standpoint, not just fit together statically. With large frames and all the variations, there are too many places for things to go wrong when you Frankenbuild them. See #2. Two of the main areas we see are a. magazines not feeding or engaging LRBHO reliably due to datum line deviations, and b. the lower bolt carrier rails smashing into the RET boss threads.
4. The upper and lower really should come from the same manufacturer, or be compatible between makers (SR-25/LaRue/LMT).
5. The barrel extension and bolt really need to be made by people who are on the same sheet of music, otherwise you will see bolt lugs shaving the extension teeth, galling, and just not interfacing with the extension properly.
6. The BCG needs to be efficient with the tri-bore dimensions in how they articulate between the carrier and the bolt body, otherwise you will have gas leakage and an inefficient system. The low and mid manufacturers deal with this by just opening up the gas port, which gasses the system even harder, and thereby causes early unlocking, mangled brass, and beats the gun up. This is hard to measure because nobody but those companies who have made their own armorer’s gauges are working off a standard. Where would you get AR-10 armorer’s gauges for the BCG, for example?
7. The chamber needs to be made for gas gun reliability and accuracy, not too tight along the case body or neck, but not too loose. Every time I sent work to GAP, they custom-reamed for me for the bullet I told them I was shooting.
8. The barrel needs to be turned, profiled, threaded, extension installed, crowned, ported, and chambered by someone that knows what they are doing and is able to reliably control concentricity and shoulder squareness for their threads and specific profile locations. They need to use a quality extension as well that is square and true with good threads, and again, proper dimensions to allow bolt articulation and lock-up. Heat treating needs to be dialed-in. I like BAT Machine extensions. There are top-tier barrel makers who have struggled with the profiling, finish machining, extension installation, concentric chambering, threading, and final operations normally done by a rifle-builder. You have to let each shop specialize in their core area of expertise, and leave the other operations to people that do those consistently well as their contribution to the manufacturing chain. GAP doesn’t make barrels, for example, but they sure know how to profile, thread, ream, etc.
9. I like blended and de-edged barrel extensions and uppers with matching surfaces between the upper receiver and feed ramps. This requires custom work to get them right. The last thing I want is the meplat getting shaved as it rides up the feed ramps. 6.5CM and .260 Rem don’t feed the same as .308 since the projectile diameters and ogive profiles are much more sleek, so it isn’t safe to assume that everything is just a barrel-swap plug-and-play. That said, my .260 Rem has never had a FTFeed, but I have almost exclusively used Gen I 7.62 NATO PMAGs that were discontinued. The lip thickness and feed angle from the mags can affect feeding. Lancers are usually a default answer if you’re having magazine feeding problems since they have nice, thin steel lips that locate the cartridge in-presentation higher than any polymer magazine. The design that has the most problems with this is the DPMS GII, since it has a smaller bolt. I wouldn’t be surprised to see similar issues with the POF Revolution, Rogue, and Ruger SFAR, since they use AR-15 sized bolts. The 2 bottom bolt lugs don’t extend down very far, so the datum lines for magazine height location have to be controlled well. Receiver slop between upper and lower can affect this too.
10. The gas system needs to start with the correct port for the projectile weight and propellant mass/burn rate you are settled into. It also needs to be located appropriately. The shorter, lighter bullets being driven by faster-burning powders can be a little closer to the chamber than the larger, heavier bullets being pushed by slower-burning powders.
Ex: 107gr with faster powder works fine with MLGS-SRLGS with 4-6” of plug dwell past the port (fore end), but 140-147gr do better with smaller gas ports and ELGS since bore volume is so small compared to .308 Win. If you want a shorter barrel, the port really needs to be small if you plan to run 140-147gr 6.5mm, especially if you suppress. I have found 123gr-130gr to work reasonably well in .260 Rem with Standard RLGS 22”, but I don’t know what port diameter GAP used. When I try to approach book max values with 140-142gr, things started getting weird with the cyclic rate, even with the buffer impacting the back inside of the RET so hard that it felt like an M203. If I tried pushing to over 2700fps with a 140gr in .260 Rem 22”, it just didn’t feel right. I could get the velocity, but brass was trashed and system behavior felt way outside of the happy window.
11. Gas block and tube need to be interference-fit, sealed, bedded, or compression-fit so there aren’t any leaks or detrimental vibration effects for precision. ArmaLite had a really slick system for their SASS rifle entry, but I haven’t seen it since. KAC’s gas system is really cool as well. I have had nothing but trouble with tiny set screw adjustable blocks. I wish Bootleg made an adjustable AR-10 carrier like they do for the AR-15. There are multiple gas tube lengths just for “RLGS” on the large frame guns. It used to be just ArmaLite (longer) and DPMS (standard AR-15 RLGS length), but has evolved. There is also ELGS of varying +_” that does well for 22”-26” 6.5-08 class of cartridges.
12. Barrel extension needs to be either press-fit, thermo-fit, or bedded into the extension tunnel in the upper if you want precision. Upper receiver face needs to be square and true, which can be off from anodizing when it dries. Extension torque is typically higher on the AR-10 at 65-80ft-lbs, but this needs to balance out with the upper receiver mass if you’re looking for optimum resonant behavior between the two components. If they are out of whack, it can show up in your groups. Scope mount and scope mass will play into this too.
13. We’re seeing a lot of cheap parts nitrided to camouflage their low quality being assembled into large frame BCGs, and they will break on you with a round count. See T.REX Arms video on the Aero .308 they tried to run for thousands of rounds. The firing pin broke, extractor pin broke, bolt catch threaded pin walked out multiple times, etc. It’s pretty sad when I look back and see DPMS BCGs as higher quality than a lot of these nitrided DIY parts that are being sold to consumers nowadays. If all you’re doing is assembling for online photos, you’ll never know, but if you actually shoot, you’ll see stuff start failing with a lot of the DIY guns.
14. I really prefer a sprung firing pin system like ArmaLite, Hk, and Savage use in their AR-10s. The floating firing pin from the DPMS design really takes a beating in .260 Rem and 6.5CM. Accelerated firing pin blowback is a thing, and I have burned through multiple firing pin retaining pins that were irreparably bent in the .260 Rem. The Savage MSR-10 firing pin seems to be the best one I have seen yet, since it’s sprung and uses the Alexander Arms triangular cross section approach to minimize any firing pin vibration in the channel when looking at timing/precision shooting. Savage MSR-10 is of course a medium-small frame AR-10, but uses standard size SR-25/AR-10 bolts in a smaller carrier similar to the Colt 901, with a forged tower base for the key instead of a carrier key. Especially if you plan to run 140-147gr or 130gr fast in a 6.5-08 cartridge, your firing pin is going to be thrust back with extreme force.
The firing pin aperture in the bolt should be smaller for this, with a small tip firing pin. These are commonly called “High Pressure Bolts”, even though it has more to do with the smaller bore volume and delayed pressure accumulation from 6.5mm and 6mm pushing long-for-caliber plugs down the pipe, with longer pressure curves. This causes added stress on the primer, which shoves the firing pin back on floating firing pin designs. For the AR-10 series, I really think a sprung pin is the way to go and should really be a standard. ArmaLite Inc. really got it right with their approach to this problem, and none of the DIY kits I’m aware of have this option.
15. Bolt catches break on large frames, especially when you run efficient suppressors, which increase your cyclic rate. Even the top names have suffered this problem, namely ArmaLite and KAC. The huge bolt carrier mass in the large frame guns really slams into the bolt catch on LRHO with far more force than any AR-15, and AR-10 action springs are generally much stiffer as well. This is why I like the Mid-Small frame as seen in the MSR-10. If you try to solve it with stronger metallurgy in the bolt catch, it will peen the bottom bolt lugs and deform the bolt, causing it to not articulate through the barrel extension.
16. For carbines, the ArmaLite mid-length (A5) RET is the way to go with a stiff action spring. Depending on what your barrel mass is and what loads you shoot, a standard AR-15 carbine buffer will work with an optimum set-up gas system that cycles inside the happy window. Many try to rate-reduce with extreme heavy buffers, but I’m not so convinced this is the way to approach this, and it’s usually on guns with too large of gas ports designed to pressure-up inefficient BCGs to get them to cycle. Multiple band-aids have been layered over the scab in those guns in my opinion, but I’m open to being proved otherwise. Rifles are easier to get to run with a Rifle RET and shorter AR-10 rifle buffer with stiff spring. The original Dutch ArmaLite AR-10s run better with a fatter BCG, fatter RET, and fatter action spring for some reason. Trey from KAC stated this and that they have learned that the SR-25 dual-OD BCG designed to work with AR-15 back-end RETs and furniture was a compromise to get rifles built with as many AR-15 parts as possible due to cost barriers for a new AR-10 in the late 1980s.
If you ever get a chance to shoot an original Dutch ArmaLite AR-10, it is a very pleasant and different experience. They are light and very well-balanced too. I love those rifles.
17. I have just used Geissele triggers and been quite happy. Before Geissele, I tried RRA 2-stage, which hung-up the BCG in the rear position until you let go of the trigger. Today, I would use Geissele, LaRue, or Triggertech I think. The factory 2-stage trigger in the Savage MSR-10 feels like an SSA or SSA-E to me.
18. Upper receivers and handguard combos that truly free-float the barrel with no contact between handguard and barrel nut can only help with precision. LaRue, Mega, Seekins, and Savage use that approach.
