This gun came out of the CMP program, and I'm the second owner. It has about 1,200 rounds through it.

I've had this rifle since 2006, and have gone from marksman to expert in NRA highpower competition.

During this time, I've used USGI LC M855 for my rapids, and Brit SS109 for standing and slow prone. The rifle functions 100% with USGI Lake City...and has short stroking issues with the Brit (and every other loading of .223 I've tried - Remington, WWB, Wolf and now Prvi). I've recently purchased 1k of 75 grain Prvi Partizan, and used that in the last match. It functioned better that most others, but still not good enough to shoot in rapids without me worrying.

About a year ago, I had it "debanned", replacing the front sight tower and threading the barrel for a flash-hider. The issues did not go away.

I'm getting low on my LC M855, and would like to resolve this issue so that I can shoot whatever I have, and specifically the Prvi 75, through the gun without worrying whether I will get to practice my immediate action skills.

I've noted no excessive/unusual wear on the BCG. Can a simple buffer spring replacement solve this? Would lighter be the way to go (or heavier because of pressure issues)? Can I snip coils off the spring, or is that bad? Or should I bite the bullet and buy a whole new b/bcg assembly? It still has the stock bushmaster 2-stage trigger in it (which does not work well with M261 .22 conversion) - could this be causing excess drag on the carrier?

Any ideas are appreciated...

This sounds like a gas system issue.  Worst case scenario is the barrel has a smaller than spec or plugged gas port.  But most likely it is further down the gas system.   These are problem areas in no special order:

Gas tube plugged....replace gas tube

Bolt carrier key plugged.....clean it

Bolt carrier key loose.....tighten it

Gas rings dry.....lube your gas rings and the bore in the carrier; make sure there are three gas rings and that they have their openings staggered

Gas rings missing or worn out.....replace the gas rings

Front sight base not lined up with gas port on barrel or may be partially plugged.....remove the FSB and check things out for alinement, possible plugging etc.

Another area not gas related, might possibly be the hammer dragging on the BCG...grind some clearance on the back/top of hammer

Actually, take your trigger assembly out and throw it away.
They're a poor design at best, and can be dangerous at their worst.

As far as the short stroking, I don't know. I have a gun doing that too, but I think it was the carrier weights I had in it.

The British Radway ammo is known to be underpowered and of questionable reliability in rifle length gas systems.

Sir:

I am not familiar with the Brit ammo but I am very familiar with the CMP AR's and who "slapped" them together. Best said, many problems! The only thing impressive on this rifle is the CMP logo! Triggers are scrap and become a hazard for range safety. Some will go good for awhile but all will eventually fail. The "glued" on front sight tower many have gas problem issues and windage  problems. Meaning, most of these CMP uppers are so far off on their mechanical zeros the shooter hasn't enough windage remaining for a 5 mph wind correction at 600 yards! Any smith that would "glue" anything on a firearm missed his calling for becoming a "used car salesman"! It's a no brainier why these CMP AR's were discontinued several years ago!
I would say, you have a gas problem. Clogged gas tube, nope I wound not go there. The gas is too hot to be a problem in the tube for carbon blockage. Check out your gas rings by doing this. Invert the bolt carrier and see if the bolt falls out under it's own weight. All should be clean before doing this test. If the bolt falls free of the carrier, your rings are bad.
Next check the carrier key. It should be staked to the bolt carrier. Any movement of the carrier key would indicate a gas problem. The barrels gas port diameter should be .0995" or use a #39 drill to correct. Sometimes a smith will not have the correct drill available and make a substitution. Wrong!
AR's are simple to work on and correcting any malfunction problems are a snap in the hands of a good AR smith. However a lot depends on the quality of the parts that were used in it's manufacturer/assembly! One reason I always recommend Rock River Arms!!!!!!! Nothing is glued on and it is backed up with a no hassle guaranty. Beat the rest by going with the best!

Dave McGrath

Try the cheap/easy stuff first,, like looking in the carrier key and making sure that ther isn't a primer stuck in there.
'Borg

Since you mention M261.
Replace the gas tube and see if the gas port needs to be cleaned out.

I have a Bushmaster DCM upper also.Check your gas tube. Thay wear out where it goes into the carrier key. This is because Bushmaster does not align the barrel nut properly.

Thanks all for the replies. I will have to check with the smith who replaced the front sight tower - he mentioned that it was originally glued on. I'm not sure whether he reglued it or not, but he put a set screw in the bottom of the bayo lug...

I tried taking off the gas tube - after getting the roll pin out, I could not get it to budge. Perhaps carbon buildup? I also could not remove the set-screw; if it's loctited, may need to heat it up? will heat harm the finish?

Bolt rings are good; it doesn't fall out of BCG under own weight. Gas key is properly staked and tight. I ran a pipe-cleaner down it, so no primers in hiding.

One thing I didn't mention previously is that the bolt face seems to have slight "crazing" of the finish - I wouldn't go so far as to say pitting, but it does tend to accumulate brass flakes that don't brush off. This was existant when I got the gun; I assumed it was carbon and would clean off at the time. Any issues with replacing the bolt on a match rifle? I.e., was it specifically fitted to this barrel by Bushmaster? It has an electropenciled "10" on it...

Unless the bolt could be an issue, my bet is that the port diameter is wrong - the gun functions perfect with known LC M855.

The trouble lies somewhere between the port on the barrel and the bolt carrier key.  I will almost bet that the way your FSB is mounted (half assed at best) that the port in the barrel and FSB are misaligned or partially plugged  or both.

Asuming that the hole in your barrel is the correct diamater, once you take off the front sight base,  you should see a carbon ring around the outside of the gas tube hole. make sure that the gas port hole is fully within this circle. It doesn't need to be concentric, just fully covered. if it's not fully covered, you (or your gunsmith) may need to cut out a little bigger hole in the FSB, so that the fsb isn't restricting the flow.

Okay. If this rifle is a CMP program AR15 rifle, guess who built them?  Don't blame Bushmaster! They furnished the lowers with the CMP logo and thats about it!

Dave McGrath

I give up.

Who built them?

oh boy!!!

The OP said...."and has short stroking issues with the Brit (and every other loading of .223 I've tried - Remington, WWB, Wolf and now Prvi). "

Doesn't sound like a specific ammo problem.

I don't know if this was already mentioned, check for loose carrier key. Two of mine eventually loosend up (bushmaster). Now I re-stake the screws on every new carrier I get.  I use center punch to deepen the staking marks that are already there.

My CMP Bushmaster went to hell in about 300rds. Loose carrier key, worn out gas rings, trigger out of adjustment...... Luckly I was at Camp Perry and Bushmaster was there. I took it to their smith and he fixed it no charge. As well as they should have.... Anyway the rifle is fine now, but get a RR match trigger.

How shocking.