AR-15 bolt carrier difference=malfunction?

alphapygmy

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I've got a new AR and old AR. The old Bushmaster DCM runs flawlessly, several times I've abused it by going over 2000 rounds between cleanings and it just keeps shooting little groups. My new AR is an CMMG 18"SPC upper and Bushy lower and I can't figure out why it short strokes about 20% of the time. I've checked for the correct buffer and spring, gas key and block leaks, it's clean and lubed properly, and is fed full powered ammo. I took both guns apart side by side to compare and feel the parts and the only difference I could find was in the bolt carrier group. The CMMG bolt is buttery smooth locking and unlocking in the bolt carrier while the old abused Bushy has a definite detent feeling as it locks and unlocks. I'm not very knowledgeable when it comes to the AR platform, can anybody send me in the right direction?
 
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Lots of new BCGs need a break-in, and/or have too much lube on them from the factory. Short stroking is the primary symptom when this happens (assuming everything else - ammo, buffer, mags -are ok).

Disassemble the bolt (ar15 dot com has some good tutorials if you need them) completely degrease the parts using any hardware store degreasing agent, or engine cleaner. Reassemble the dry parts, and hand cycle the bolt 100 or 200 times.

Last and not least - apply a very small amount of lube (teflon is a good one) then go back out and shoot it again. The last bolt I needed to break-in took about 500 rounds - but when finished it shot great.

Hang in there. Let us know how it goes.

Nate
 
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It's hard to fathom why a new acrrier group woulds hang up with full power loads but:
Check that gas ring gaps are staggered
Check for burrs on all surfaces
Check for obstruction in gas system
check mag. swap mags, load 2 rounds under stated cap.
Last resort send upper back to MFG
 
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alphapygmy

alphapygmy

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When it does eject they go between 2:30 and 3:30. I'll clean, check, and cycle the whole BGC tonight and see what happens. If it is overgassing I can put my carrier weight system from my match gun in the new one and see if it changes anything. I might swap the whole BCG too, if I get desperate enough.
 
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alphapygmy

alphapygmy

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Hopefully I found the problem. I disassembled and cleaned/checked almost everything-bolt, carrier, gas block and system. Everything looked like it was supposed to. I even swapped in the BCG from my Bushmaster to try things. Nothing worked. I did the fire one round test and it failed to lock the bolt back almost every time. My conclusion was the Wolff extra power buffer spring was causing the short stroking so I have a good quality standard rate buffer spring on the way. After concluding that was the likely problem I started running through some forums and there seemed to be a consensus that extra power buffer springs in rifle length gas systems cause alot more trouble than they're worth. I checked my old parts receipts and found I had ordered one for both my AR's so that would explain why my Bushmaster lower didn't work with the CMMG upper. I have all my buffer springs now "quarantined" and if the new spring solves the issues I'll toss the old ones and order standard ones for both guns and a spare for each. Fingers crossed and I'll post the results when the new spring gets here. Thanks for the ideas, it's been frustrating but I've learned a ton I wouldn't have otherwise.
 
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alphapygmy

alphapygmy

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Sigh....Tried the new buffer spring and it still short strokes. Felt a big difference in the hammer spring so I changed that to a standard one and it didn't help. Double checked my old Bushmaster with the same ammo and magazine and it worked fine. Then I took it's lower and put it with the CMMG upper and got a different failure! It jams with a spent case just making it out of the chamber and the new round trying to get in at the same time. I'm going to check the gas key for leakage but after that I'm fresh out of ideas.
 

DirectDrive

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Sounds like you have done almost everything in your troubleshooting process except :

1. Identify your ammo....I would run some XM193 or XM885 through it if you haven't already.

2. If the above full power ammo doesn't cycle it, thoroughly investigate your gas system starting at the gas block/gas port/gas tube. Leaks ? Proper alignment ? Is the port clear ? What is the diameter of the port ?
 
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I recently had a similar situation. I replaced the BCG in one AR with an M-16 BCG. Short cycled every round. The M-16 BCG is about 30% heavier than the AR-15 BGC. It ran fine with 77 grain ammo, but would short cycle on 55 grain. So what we did, (we just happened to be at a gunsmith with a shooting tank) was to remove rings on the stock spring to reduce tension. It took several try's but after about 4-5 rings removed it cycled perfectly, using both 55 and 77 grain ammo. Had nothing to do with the mags, or the gas system. Just the extra weight of the BCG and the spring tension. It now fires well, with either BCG installed.
It also could be that after about 500 rounds (yet to be done) that it will break in a bit, as mentioned above. It did take a bit of fine tuning though.
 
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alphapygmy

alphapygmy

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Sounds like you have done almost everything in your troubleshooting process except :

1. Identify your ammo....I would run some XM193 or XM885 through it if you haven't already.

2. If the above full power ammo doesn't cycle it, thoroughly investigate your gas system starting at the gas block/gas port/gas tube. Leaks ? Proper alignment ? Is the port clear ? What is the diameter of the port ?
So far it's had the same symptoms with XM193, plinker Rem. 55gr FMJ, and my own 75rg load that closely copies the Mk262 loading. I went through the gas system, the gas block is aligned and not leaking around the barrel, gas port measured ~.090", supposedly typical for a rifle length system. Gas key on BCG was nice and tight still. The only thing that stuck out was a little leakage around the gas tube going into the gas block and the gas tube roll pin.
Like the previous poster mentioned, I'll look into the possibility of having an M-16 BCG. I'm going to try posting over at ar-15.com and see what they think as well.
 
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I could be way off base here, because I know the barrel probably came assembled with the upper, but couldn't the problem be an 18" barrel with a rifle length gas system? Maybe the dwell time is not close enough to optimal for it to work right 100% of the time. Here is a lengthy write up: How it Works- The AR gas operation and how everything works in harmony. - AR15.COM According to this article a mid-length system is closer to optimal for an 18" barrel.
The easiest test I can think of would be to screw the longest/ least slotted flash hider on there you can find and see if the extra length helps. I also think a mis-aligned gas port to gas block is a likely culprit, especially if it's a clamp on style block.
If the problem is the dwell time, maybe look into enlarging the gas port? I don't know if that's a feasable solution, but short of sending the upper back or putting a long, custom flash hider on, it may be.
 
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Wait, let me guess... 18" barrel with a rifle length gas system according to your post above?
Notorious for exactly what you describe unless running a suppressor or really hot ammo.
Not enough barrel in front of the gas port to properly run the impingement system.
IMHO, 18" w/rifle length is intended for piston kits or suppressors only.
My 2 cents. (based on the exact same experience.)
 
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There are a few things you should check first,

.make sure the propper buffer & spring is in there, i.e factory
. remove front sight and inspect gas port.
. inspect gas tube, make sure it's in propper.
. inspect gas rings make sure there good and timed propper
. make sure all parts move free with out lube.
. check the caseings when it fails what color is the empty case?

i can go on forever, if you still have problems with it, bring it by the shop we'll take a look at it.
 
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alphapygmy

alphapygmy

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I could be way off base here, because I know the barrel probably came assembled with the upper, but couldn't the problem be an 18" barrel with a rifle length gas system? Maybe the dwell time is not close enough to optimal for it to work right 100% of the time. Here is a lengthy write up: How it Works- The AR gas operation and how everything works in harmony. - AR15.COM According to this article a mid-length system is closer to optimal for an 18" barrel.
The easiest test I can think of would be to screw the longest/ least slotted flash hider on there you can find and see if the extra length helps. I also think a mis-aligned gas port to gas block is a likely culprit, especially if it's a clamp on style block.
If the problem is the dwell time, maybe look into enlarging the gas port? I don't know if that's a feasable solution, but short of sending the upper back or putting a long, custom flash hider on, it may be.
I just found the same graph and chart on weaponevolution.com. I think the chart has the answer, 18" barrel + rifle length gas system=.131 milliseconds, not enough time for enough gas to operate the gun. So having checked with known good mags, known good ammo, cleaned and oiled everything, checked gas key, block alignment, tube alignment, gas rings, buffer and spring, and everything else everybody recommended I'm going to assume I'm not getting enough gas. Since there is no chance of talking my wife into a suppressor I'll give CMMG a call next week and see if they recommend opening up the gas port a little or what.
 
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Contrary to popular belief, staggering the gas rings does nothing. They can all be lined up and the gun will still function.
If the gun will not function with the rings aligned, there's something else to look for.

As the carrier cycles the rings will rotate, it happens, and there's nothing to stop them from doing so.
 
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alphapygmy

alphapygmy

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Contrary to popular belief, staggering the gas rings does nothing. They can all be lined up and the gun will still function.
If the gun will not function with the rings aligned, there's something else to look for.

As the carrier cycles the rings will rotate, it happens, and there's nothing to stop them from doing so.
I wasn't worried about the gaps lining up, just the rings being broken or too loose.
 
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alphapygmy

alphapygmy

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SUCCESS!! Opened up the gas port from .093" to .1015" and it runs flawlessly now. Ran through a couple hundred rounds of different ammo and even loaded up some low pressure rounds to see how they would work. Everything from light to hot loads ran perfect with no short stroking and no battering from the buffer cycling back too far in the buffer tube. Thanks all!
 
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SUCCESS!! Opened up the gas port from .093" to .1015" and it runs flawlessly now. Ran through a couple hundred rounds of different ammo and even loaded up some low pressure rounds to see how they would work. Everything from light to hot loads ran perfect with no short stroking and no battering from the buffer cycling back too far in the buffer tube. Thanks all!
Nice! That's a relief huh?! Did you end up calling CMMG?
 

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