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 ftf and not locking back?
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Cooperarms
New Member

USA
29 Posts

Posted - Jun 03 2012 :  21:41:32  Show Profile Send Cooperarms a Private Message  Reply with Quote
Hey guys, I just put a 20" DPMS bull barrel on my stock 16" carbine AR, with new gas block, rifle length gas tube and a rifle length free float barrel. I love the accuracy improvement. However, occasionally it fails to feed (never failing to eject) and occasionaly does not Lock back on the last shot. The gas block is still locked down tight. A guy at the range said I might need to get a new buffer tube due to the modification in barrel length. Im mostly shooting hand loads (have never had a problem with them before now), but have shot a few factory, same issues.
Like I said, its not happening often, just enough to concern me.
Any ideas on my problem?
Thanks!

Coop

DocHolladay
Advanced Member



USA
1299 Posts

Posted - Jun 03 2012 :  23:52:43  Show Profile Send DocHolladay a Private Message  Reply with Quote
If I changed anything, I would try a different buffer. The one you have may be to heavy. What weight do you have now? H1, H2, H3?

Welcome to Tennessee, the patron state of shooting stuff!
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Cooperarms
New Member

USA
29 Posts

Posted - Jun 04 2012 :  00:33:27  Show Profile Send Cooperarms a Private Message  Reply with Quote
To be honest, I don't know the weight. Is it indicated on the buffer?

However, I did move from 52 - 55gr. (which is what I was having the problems with) to 75 BTHP and I had no FTF's or failure to lock back on the last shot. For what its worth.

I will check the the weight. Is H1 the lightest or the heaviest (probably a dumb question)?.

Coop

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Shastaboat
Advanced Member

USA
5457 Posts

Posted - Jun 04 2012 :  00:55:44  Show Profile Send Shastaboat a Private Message  Reply with Quote
What is the 55 gr load you are using? May be too light.
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anewname
New Member

USA
37 Posts

Posted - Jun 04 2012 :  07:12:03  Show Profile Send anewname a Private Message  Reply with Quote
I recently had a similar issue with not locking back when the last round was fired on a brand new upper. I took it to a local gunsmith after I noticed quite a bit of fouled gas coming out from under the gas block. I have a stainless HBAR, so it was pretty easy to see on the barrel, if yours is CM you might have a harder time noticing.

The smith told me that sometimes if a gas block is not super tight, it will take a few hundred rounds under the block to "gum up" and create that good seal. After I put another 40 plus rounds through the barrel, the problem stopped.
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Cooperarms
New Member

USA
29 Posts

Posted - Jun 04 2012 :  19:50:55  Show Profile Send Cooperarms a Private Message  Reply with Quote
quote:
Originally posted by Shastaboat

What is the 55 gr load you are using? May be too light.


Actually, the 55 gr. were factory Hornady Superformance V-max. My handloads were Hornady 52 gr. BTHP with 24gr. of H335, and Hornady 53 gr. V-max, also with 24 gr. of H335.

Coop

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DocHolladay
Advanced Member



USA
1299 Posts

Posted - Jun 04 2012 :  23:01:03  Show Profile Send DocHolladay a Private Message  Reply with Quote
H1 is usually the lightest and H3 is the heaviest. Weights vary by manufacturer.

Welcome to Tennessee, the patron state of shooting stuff!
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Quack Addict
Senior Member

USA
210 Posts

Posted - Jun 06 2012 :  14:03:31  Show Profile Send Quack Addict a Private Message  Reply with Quote
To the OP, you may want to try changing your buffer spring.

I'm no expert but I thought a "rifle" buffer was a rifle buffer.

Unless I'm mistaken, the H-buffer terminology applies to carbine buffers. Rifle and carbine buffers are NOT interchangeable!

Rifle buffer = 5.1oz with 3.5oz reciprocating mass

Standard carbine buffer = 3.0oz with 1.9oz reciprocating mass
H1 carbine buffer = 3.7 / 2.8
H2 carbine buffer = 4.6 / 3.5
Not sure on the specs of an H3 carbine buffer.



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dh2
Advanced Member



709 Posts

Posted - Jun 06 2012 :  22:05:14  Show Profile Send dh2 a Private Message  Reply with Quote
by hodgen web sight your 24.0gr of H335 is the starting charge I would try working up with the powder charge to the max of 26.0gr
I was told when building my first ar-15 that useing a m-16 bolt carrier would help elimanate feeding problems ,so I have used the heaver bolt in 4 I have and do not have this problem, so it may help I have not built any with a carbine buffer, except for a 9mm it is adiffrent system all togather, I have one with a 24" bull barrel on the work bench it will get a m-16 bolt carrier too.
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Quack Addict
Senior Member

USA
210 Posts

Posted - Jun 07 2012 :  23:39:52  Show Profile Send Quack Addict a Private Message  Reply with Quote
quote:
Originally posted by Cooperarms

carbine AR
new gas block
rifle length gas tube
occasionally it fails to feed (never failing to eject) and occasionaly does not Lock back on the last shot



Just re-read the OP.

1. I'd recommend trying an H2 buffer, runs about $30. It will plug & play in your carbine buffer tube assuming you do in fact have a carbine buffer tube. A 'regular' buffer is too light for some AR's running rifle length gas systems - end result is fail to feed and failure to lock back on an empty mag. IF YOU HAVE A RIFLE BUFFER TUBE, don't try putting in a carbine buffer in it. You and your rifle will probably end up broken due to BCG over-travel.

2. There is a possibility the new barrel gas port isn't opened up to spec. I think the hole is supposed to be .090" (but research that before you get the drill out). You can use the unfluted end of an appropriate size drill bit as a gauge - use a micrometer or calipers to measure the bit. Also, the gas block may be acting as a choke-point for the gas if the path through it is constrictive (too small). A burr (or burrs) in the gas block could be culprit as well. Off chance the gas port in the barrel and path in gas block aren't lined up properly. Long shot is the gas tube is plugged or obstructed somehow, or dirty. Remember the pipe cleaners kids play with...?

3. Buffer springs are cheap. They can be had for about $3, a good one for about $15.

I don't think the problem is with your ammo. I do agree that a heavier (M16 / full auto BCG) will help and make for more reliability but I think more effect will be had by changing buffers (much cheaper too).

I'd be interested in your solution if you don't mind posting it after you resolve the issue.

Pic of different BCG's. Notice the M16 / full auto BCG has more mass. Unless you are in Kalifornia, DC or other gun-unfriendly place a full-auto BCG should pose no legal issue.




Edited by - Quack Addict on Jun 07 2012 23:45:31
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Quack Addict
Senior Member

USA
210 Posts

Posted - Jun 13 2012 :  18:42:52  Show Profile Send Quack Addict a Private Message  Reply with Quote
I just finished a rifle with the same basic setup as in the OP (rifle length gas system and carbine buffer setup) and it was also failing to feed from the magazine (sometimes snapping shut on an empty chamber, sometimes jamming the round in the receiver after skipping off the rim) and not locking back. After playing around with the buffer and spring with no success I ended up opening the gas port on the barrel. It started .092" and the second iteration left me at at .100" it now cycles fine. The rifle has a H2 buffer in it but there was little if no effect with the standard buffer installed. It's keeping the H2 for now as it should help decrease component wear. May need to reevaluate that in cold weather this winter.

One other thing to check is gas block alignment to the gas port in the barrel. Saw one recently where the gas block was rotated a fair amount. If bad enough the gas block could obscure the gas port in the barrel.



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