Built new AR-10, light-weight ammo returns poor results

[VPSowner]

Just got in the last of the parts to put together my first .308.  Took it to the range today, function was good, but accuracy was sub-par.  Most of that is probably due to the ammo, which I fully recognize was a poor choice, but I've had better results in another rifle with the same twist.

I was mostly using Magtech CBC 147grn 7.62x51mm Brazilian made ammo.  Accuracy at 100 yards was disappointing, shots formed a circle about 4" wide.  Several times the rifle would fail to feed, the round was left in the magazine, and the bolt traveled right over it.  Fired about 30 shots, had four of these failures to strip the next round from the mag.  I can't recommend this ammo, even though it's brass case, it's not terribly consistent.

Also used Tula 150grn .308 for 10 shots.  Every round functioned perfectly, but accuracy was still about 2-3" wide circle at 100 yards.  I did not bring any heavier bullets out, I was just trying to break the rifle in some, and I don't have a lot of the 180grn to plink with.

The rifle is built using a CMMG lower, LPK, bolt group, and 18" SS barrel (mid-gas, 1:10" twist).  DPMS upper, Troy gas block and rail, PWS FSC muzzle brake.  Heavy weight buffer and .308 carbine buffer spring.  Never had failures to eject, and recoil is very light (less noticeable than my piston-driven 6.8SPC).

I have a Remington 700 AAC rifle with the same 1:10" twist, it will return respectable groups with the Tula 150grn .308, even though it prefers the 180grn heavier loads.  I'm hoping this rifle tightens up some when I'm able to feed it better ammo, even as a dealer, it's hard to order ammo these days.

[MikedaddyH]

Welcome from AZ !

Just need to know what magazine you used ?? This maybe the first thing to look at ( Steel mags hang up rounds sometime, pmags should solve this problem.) Pick up a box of 150gr FMJs and go back to the range. Lastly Run that Bitch wet ,Gun lube - Moblie One - M7. Dripping WET ,oil running down your arm WET !

Do this until its broken in.

[VPSowner]

Forgot to add the mag details.  I was using a Magpul Pmag, although I'd like to try some other DPMS compatible magazines down the road just because. 

While not really interfering with the rifle functions, I did have one issue with the Pmag.  There were a few times the rifle would lock back on an empty mag, but the bolt was only caught on the magazine follower, not the bolt catch.  The CMMG bolt catch can find a way to slip behind the follower, forcing the follower forward and up to catch the bolt.  I've ordered a DPMS bolt catch, thinking the extension might be long enough to assure better contact.  It only did this using the 147grn 7.62x51mm, the Tula 150grn .308 always functioned properly.

While not a knock on CMMG, I've had this bolt catch issue happen on an AR-15 built with their lower parts kit before.  One just did not make good contact with the back of the follower, and needed to be replaced.  I actually have a lot of respect for CMMG, and it's just a coincidence that they were supplied with a part that didn't meet spec, that's not something they make themselves.

[texas30cal]

I had similar results with my steel barrel when brand new and the more I shot it the worse it got, ( 30 or 40 rounds), I went to the range with a good friend and he asked if I has cleaned it yet, I said I had only cleaned it with hoppes and a jag, He had some bore tech eliminator, we used it on a plastic jag until no more blue came out on the patches, the blue is a indication that it is removing copper,  then hit it with the hoppes. It improved immediately! Then we started shooting 5 or 6 rounds and repeating this procedure, did this for 60 rounds or so and now it shoots way better than I can with bad eyes and carpal tunnel in both hands. It may not be your problem but that stuff worked very well and would be worth a shot. As far as breaking the rest of it in I use moly grease on the bolt lugs and oil on everything else.

[unforgiven]

Welcome from Indiana brother.

[Matt.Cross]

I recommend withholding judgement on the rifle until you feel like you've got it broken in and start shooting match ammunition through it. A bolt rifle is very forgiving in comparison and you're not the first to note how much more forgiving they are with mil-surp. Shoot some FGMM or comparable match ammo through it, and focus very heavily on your fundamentals, then make your judgement call on the rifle. It would be a shame to make a poor call on that rifle based on it's performance with mil-surp.

[imschur]

I like to quote JP from an email to me

Many people think that just because some factory makes a round in their caliber (say 308 or 7.62), it should function in any rifle. Nothing could be further from the truth.  There is a very wide range of internal ballistics on all this ammo and it is not realistic to expect that a self loader will function with any round out there.  Weapons designed for military applications assume that they will use military grade ammo which will in fact fall into a relatively narrow range of specifications, both dimensionally and in terms of internal and external ballistics.  So it follows that you can have a fixed gas system that will deliver a specified reliability requirement as long as you use the required ammunition. 

However, take the M118LR for example.  This is purpose specific round that exceeds SAAMI spec on the OAL and was never meant to be used in a gas gun but yet, we use it even though it will not feed from all magazines. 

[VPSowner]

I appreciate the replies. As to the moly-grease on the bolt lugs, is that only during the break-in period, or is that something you do during your normal maintenance during the entire service life of the rifle? One last thing. I forgot I had a POF roller cam at the shop, and threw it into my bolt carrier after it's first range trip. I have confirmed with POF that its compatible with my build, but was curious if anyone else had results from making that swap. It has made my bolt noticeably easier to cycle, but I'm worried it might provide too little resistance for long-term use. I've just got it oiled now, but I'm considering using the moly grease on the roller cam as well, at least during break-in.

[texas30cal]

Mine was tighter than I thought it should have been when locking and unlocking and it would not completely lock closed sometimes, I read about the moly grease on here somewhere, I put enough on the lugs to cover them with a small brush and kept it there until it worked freely now all it needs is a light coat of oil.

[survivalshop]

I also have a CMMG lower with a SI Defence upper & it's an older CMMG lower. This lower will not reliably function with the PMag. Same as you have described , bolt over &failing to strip & feed the next round. My rifle functions flawlessly with CProducts or DPMS mags. I also have the POF roller cam pin & it works fine. As far as your groups , break the barrel & rifle in with good ammo & they should get better . A good trigger will also help.