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Old 09-19-2013, 05:16 PM   #1
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Default Ar15 ftf issues.

Okay so I bought a used older colt upper with bcg for a song last year. Decided to build my lrb lower I had laying around and use it as a beater/plinking gun. I assembled it with the old colt a1 fixed carry handle and a offset bushing. It was rattled but ran fine. Decided to go to a flat top stag upper and keep the barrel assembly. Performed the swap without removing the gas block (I did replace the gas tube though, rifle length system). Now I'm getting ftf issues. Tried multiple mags including a new brownells mag and a new pmag. Ejects every time but sometimes does not pick up a new round, or only partially loads it(wedging). If I pull the ch all the way back it loads fine. Also does not lock back on empty. I think I was able to get 4 rounds to cycle but usually it was every round.

No scoring or marks on the bcg, again the rifle ran fine before and I only removed the gas tube and reinstalled. Ammo used is American eagle 55grn factory brass loads.

Rifle:
Stag arms upper receiver (new)
20" colt barrel assembly (used)
Lrb lower with bushmaster parts kit
Colt bcg and ch
Stag buffer tube and buffer/spring

Symptoms make me think a gas problem, but since I never removed the gas block I doubt it's that. Gas key is staked and solid with no movement. Should I replace the buffer with a lighter one? Can new uppers really be this finicky during break in? (Only fired about 50 rounds). I lubed with rem spray since my good stuff is at work. I'm going to try to clean relube and try again. Any other suggestions?

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Old 09-19-2013, 05:41 PM   #2
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Guns can be very finicky during break in. It sounds like a combination of issues. I'd definitely double check the gas system. Make sure the bolt is assembled correctly and run it wet. Running it wet may solve the cycling issue until things loosen up. If not a lighter buffer may be in order.
Are the mags you're using proven? Have they worked reliably in the past?
What kind of ammo are you using? Breaking in with steel can be a nightmare.

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Old 09-19-2013, 06:12 PM   #3
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So, you transferred the barrel and gas block onto a flat top upper reciever?

If you only load one round into the magazine, and fire it, does it lock back the bolt?

It honestly sounds like its a bit undergassed. What weight buffer are you running? Is it a rifle buffer system or carbine?

Easy solution, lube the bcg and lube the buffer and spring, check the gas rings, and then try it, and try the bolt lockback test. If its still not locking the bolt back try a lighter buffer. If that doesn't work, time to remove the gas block and tube, clean them out and re-install them.

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Old 09-19-2013, 08:16 PM   #4
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Quote:
Originally Posted by red ryder View Post
Okay so I bought a used older colt upper with bcg for a song last year. Decided to build my lrb lower I had laying around and use it as a beater/plinking gun. I assembled it with the old colt a1 fixed carry handle and a offset bushing. It was rattled but ran fine. Decided to go to a flat top stag upper and keep the barrel assembly. Performed the swap without removing the gas block (I did replace the gas tube though, rifle length system). Now I'm getting ftf issues. Tried multiple mags including a new brownells mag and a new pmag. Ejects every time but sometimes does not pick up a new round, or only partially loads it(wedging). If I pull the ch all the way back it loads fine. Also does not lock back on empty. I think I was able to get 4 rounds to cycle but usually it was every round.

No scoring or marks on the bcg, again the rifle ran fine before and I only removed the gas tube and reinstalled. Ammo used is American eagle 55grn factory brass loads.

Rifle:
Stag arms upper receiver (new)
20" colt barrel assembly (used)
Lrb lower with bushmaster parts kit
Colt bcg and ch
Stag buffer tube and buffer/spring

Symptoms make me think a gas problem, but since I never removed the gas block I doubt it's that. Gas key is staked and solid with no movement. Should I replace the buffer with a lighter one? Can new uppers really be this finicky during break in? (Only fired about 50 rounds). I lubed with rem spray since my good stuff is at work. I'm going to try to clean relube and try again. Any other suggestions?
First, I would replace the gas rings and extractor spring assy. Next do a barrel nut alignment test. To do this, leave the bolt out of the carrier, put the carrier in the receiver, turn the upper so the barrel is pointed straight up, and slowly push the bolt carrier up. You should just barely feel when the gas key contacts the tube, if at all. If you feel a hard contact, you need to remove the gas tube, then back the barrel nut off and re-tighten it. Brownells has a metal rod that is slightly larger diameter than the gas tube to insert into the gas key. When you are tightening the barrel nut, if the rod goes between the teeth of the nut, then the gas tube surely will. If you have to re-tighten the barrel, it can make sure the gas tube freely moves through the barrel nut.
As far as the BCG not locking back on an empty mag, it sounds as though the spring is too stiff on the bolt catch. You can take a pair of dykes and clip it one loop at a time until you get the right tension.
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Old 09-20-2013, 01:57 AM   #5
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20" colt barrels typically do not have m4 cuts. this can be a real issue when you install them into flattop uppers as almost all have m4 cuts and can cause severe fail to feed.


second if you have installed one of those crappy bad type levers thats almost certainly whats causing your fail to lockback.

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Old 09-20-2013, 02:32 AM   #6
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Quote:
Originally Posted by JonM View Post
20" colt barrels typically do not have m4 cuts. this can be a real issue when you install them into flattop uppers as almost all have m4 cuts and can cause severe fail to feed.


second if you have installed one of those crappy bad type levers thats almost certainly whats causing your fail to lockback.
Nice pickup JonM, I didn't even think of the feedramp issue since most barrels nowadays have m4 feedramps. That is a very important factor!
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Old 09-20-2013, 12:07 PM   #7
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Feed ramps are fine. Rifle extension in a rifle receiver. Cleaned and lubed last night. It feels better.

How do I tell which spring/buffer is in there? I just threw on a buffer tube spring and buffer that I had from a gun that I upgraded the stock on. Going to try running it wet before I make any mods.

Mags are proven reliable.

Very slight contact between gas tube and gas key. Does not seem to be significant. I aligned the gas tube when tightening the barrel nut and the tube moved freely.

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Old 09-20-2013, 04:28 PM   #8
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You check the gas rings on the bolt?

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Old 09-22-2013, 05:43 AM   #9
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Yup gas rings are fine

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Old 09-22-2013, 10:51 AM   #10
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My general practice is to use the spring/buffer that came with the buffer tube, usually just going with a buttstock assembly. Trying to play around with different buffer weights is outside my current skills. I think Tikki has more experience in that area. That way I can send anything that doesn't work out to the manufacturer without them bitching about me modifying anything.

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