Ar-15 ftf fte issue

[Guest sL1k]

I recently built an ar15 pistol with a 7.5inch barrel on an nfa polymer lower. I ran 90 rounds of xm193 through it with no issues a couple months ago. Didnt clean it and took it shooting the other day. The first failure was a completely seated round but the trigger would not depress (Im guessing the bcg is sliding back just far enough to strip a new round but not reset the trigger? ) then I started to experience the ftf/fte as shown in the picture. Im using a phase 5 pistol buffer.


The brass is ejecting to the 1-1:30. And the pictured failure occurred once every 10 rounds the trigger issue was about once every 20. Tried 3 different magpul mags with the same issue. After I experienced the failure twice I pulled the bcg lubed it inspected gas key for being loose reinstalled and still had failures.

Used 2 different batches of xm193 ammo to rule it out. And all ammo mags worked well in my cmt gas piston rifle.


The upper was bought complete from red x, bcg is a cmt tactical.

Any ideas?

[Oh Shoot]

I:00-1:30 would suggest significant OVERgassing rather than short stroking, and is common with sub 10" barrels.

 

Would also explain the jams, but not sure regarding trigger reset.

 

- OS

Edited October 2, 2013 by Oh Shoot

[orientalexpress]

Try running a heavy buffer or a stronger buffer spring. Possibly a combination of both. I'm not familiar with a "pistol" buffer. Is it heavier or lighter than a standard?

[KKing]

It's over gassed man. Heavy buffer, maybe even a 9mm buffer. I'd take it down and put an adjustable gas block on personally

[Guest summertime27]

 I'd take it down and put an adjustable gas block on personally

 

Ditto This!

[Guest 6.8 AR]

When it ejects, it should have already reset the trigger. Look for something faulty in the trigger group. Maybe a broken

or faulty catch on the hammer. It could have been broken all along, or a spring in backward. I'd only do the adjustable block

if I was going to suppress it. The buffer should do the trick.

[TMF]

Yeah, the trigger reset issue doesn't seem to make any sense. So I'm assuming that what you're describing is the rifle cycles the bolt, ejects the round and when you go to pull the trigger nothing happens? Is the hammer cocked or forward at this point? If it's forward I think you're lucky you're not having a slam fire.

[Guest sL1k]

Yeah, the trigger reset issue doesn't seem to make any sense. So I'm assuming that what you're describing is the rifle cycles the bolt, ejects the round and when you go to pull the trigger nothing happens? Is the hammer cocked or forward at this point? If it's forward I think you're lucky you're not having a slam fire.

It feels just like the safety is on is the best way to describe it, going pull the lower down and make sure nothing is broke.
I will try a heavier buffer before I go shoot again that would make sense since it ran good the first 90 rounds with a new buffer spring.

[TMF]

It feels just like the safety is on is the best way to describe it, going pull the lower down and make sure nothing is broke.
I will try a heavier buffer before I go shoot again that would make sense since it ran good the first 90 rounds with a new buffer spring.

Hmm. Is the bolt all the way seated when this happens? Also, is the LPK also plastic?

[Guest sL1k]

Hmm. Is the bolt all the way seated when this happens? Also, is the LPK also plastic?

It is all the way seated slammed the forward assist to be sure. The round ejected easily when I pulled the charging handle. The lpk is a cmmg.

[TMF]

Off the top of my head I can't figure why. I would have to actually look at it. I'd say next time you take it shooting, when it fails to reset go ahead and remove the lower and inspect the trigger group.

[RWF]

if the trigger is locking up, as you stated its like the safety is on. look for a blown primer or = trash under or around the trigger. ask me how I know. on the other I would first say a bad mag, but you say that is not it. I really do not know about those very short barrel lengths but inspect your bolt and be sure the ejector is free and working and the extractor is not broken and the you good spring tension.

 

just guessing

[Dolomite_supafly]

Try looking under the trigger for a primer. Another issue could be the hook of the disconnect is too long and not releasing the hammer when you release the trigger. Not sure if you remember but do you remember hearing a click when the trigger was released. I have seen this on quite a few rifles over the years. The way to fix it is take a file to the front of the disconnect hook. If it happenes again take your finger and press forward on the trigger. If the trigger clicks and the gun is able to be fired it is the disconnector hook that is too long.

 

What do you have for a muzzle device? Certain muzzle devices can increase pressure and cause problems on these short guns.

 

I tell anyone, that will listen, to install an adjustable gas block.

 

And finally, give it a good cleaning. Make sure the ejector spring has the black or blue insert inside the spring. Excercise the ejector too while oiling it.

 

Have you tried a different lower? My frontier lower seems like it is slicker than a standard lower. I had an upper that would not cycle using standard lowers but would cycle with the frontier. That tells me there is less resistence with the plastic hammer. Could be the less resistence is contributing to the overgassing issue. An adjustable gas block will fix all the over gassing issues.

Edited October 2, 2013 by Dolomite_supafly

[Guest sL1k]

Off the top of my head I can't figure why. I would have to actually look at it. I'd say next time you take it shooting, when it fails to reset go ahead and remove the lower and inspect the trigger group.

It had me puzzled too especially with the bolt all the way seated (my first thought was that it wasn't seated)

if the trigger is locking up, as you stated its like the safety is on. look for a blown primer or = trash under or around the trigger. ask me how I know. on the other I would first say a bad mag, but you say that is not it. I really do not know about those very short barrel lengths but inspect your bolt and be sure the ejector is free and working and the extractor is not broken and the you good spring tension.

just guessing

I will check for trash too!

[Guest sL1k]

Try looking under the trigger for a primer. Another issue could be the hook of the disconnect is too long and not releasing the hammer when you release the trigger. Not sure if you remember but do you remember hearing a click when the trigger was released. I have seen this on quite a few rifles over the years. The way to fix it is take a file to the front of the disconnect hook. If it happenes again take your finger and press forward on the trigger. If the trigger clicks and the gun is able to be fired it is the disconnector hook that is too long.

What do you have for a muzzle device? Certain muzzle devices can increase pressure and cause problems on these short guns.

I tell anyone, that will listen, to install an adjustable gas block.

And finally, give it a good cleaning. Make sure the ejector spring has the black or blue insert inside the spring. Excercise the ejector too while oiling it.

Have you tried a different lower? My frontier lower seems like it is slicker than a standard lower. I had an upper that would not cycle using standard lowers but would cycle with the frontier. That tells me there is less resistence with the plastic hammer. Could be the less resistence is contributing to the overgassing issue. An adjustable gas block will fix all the over gassing issues.

I do not remember the click. But If it is a disconnect issue I should be able to replicate it without firing it. So I will check that also. It has a birdcage on it I thought about getting the dpms levang linear, im guessing that idea is out until I get an adjustable block. I do not have a spare lower to swap everything over to. I guess I could have taken the stock off my cmt and tried it at the time but I had this feeling the ATF was going to speed rope out of helicopters no matter how much tinfoil I was wearing.

What brand adjustable block would you recommend?

And thank you guys for all the responses and help!

This was a complete low budget build. I set out to see how low I could build an ar15 pistol for in the middle of a panic, and to keep with that spirit im going to see the lowest cost I can repair it for be it buffer or gas block and by pickin yalls brains. ( not to say i won't spend whatever needed to make it right). I have actually gotten a couple people at work interested in building their own/first ar15 by showing them this one and letting them know how much it cost.

[Guest 6.8 AR]

Nothing wrong with adjustable gas blocks, but it isn't necessary when you can fix the problem cheaper and just as effective

by playing with different buffers and springs. The gas block only adds weight. Both of mine only got pistol buffers, but

my barrels are longer. I have never had a gas problem with them. As far as on the muzzle, try a Noveske pig, if you can find

one. They are great, and a lot of the sound is projected away from you(forward).

[Guest sL1k]

Nothing wrong with adjustable gas blocks, but it isn't necessary when you can fix the problem cheaper and just as effective
by playing with different buffers and springs. The gas block only adds weight. Both of mine only got pistol buffers, but
my barrels are longer. I have never had a gas problem with them. As far as on the muzzle, try a Noveske pig, if you can find
one. They are great, and a lot of the sound is projected away from you(forward).

I really want a pig but just cant justify it.

[Guest 6.8 AR]

Oh, yes you can. Trust me, you'll find a way. :D

[Dolomite_supafly]

A PIG will increase the pressures in the barrel and only increase the over gas issue.

They make micro gas blocks that are adjustable.

Battle Tested Equipment, BTE, makes a SS micro block that is adjustable for $49 shipped.

He might need to buy several buffers to find one that works. Then you end up with a lower that may not work with other uppers. It is always better to fix the upper and leave the lower alone so you can swap the upper and lower onto different uppers and lowers.

[Guest 6.8 AR]

I doubt he will be using many different uppers on that pistol lower, but I digress. With a pistol lower, you won't get

too much out of whack by changing buffers for all the barrels you can find for a pistol. It can be a much broader range

with a rifle.

I may have to get one of those gas blocks and try it out.

 

Do you have some pig data on pressures, Gordon? I can't see how that thing raises much in pressure. There's almost

nothing to them, except that cone.

[TripleDigitRide]

Man, threads like this make me nervous to build my 300 BLK pistol. I plan(ned) using a PIG on mine, mainly because I like the way it looks.

[Dolomite_supafly]

It is like a suppressor. I spoke to John Noveske about 8 years ago on the phone about the KX3 when I called to order one. He explained to me about the pressure increases then and how it increased reliability in some guns because of additional pressures. I know a few smiths that use it to diagnose problems.

The shorter the barrel, especially pistol length, are a lot more finicky and even minor changes can have dramatic effects on durability. Rifle length barrels tend to be less affected by changes because of the longer gas impulse.

[Lumber_Jack]

Man, threads like this make me nervous to build my 300 BLK pistol. I plan(ned) using a PIG on mine, mainly because I like the way it looks.

556 and 300BLK are different animals. I would put (and did) an adjustable gas block on a .300 from the word jump.

I also put a pig on mine just to help increase pressure for subs

[RWF]

It is like a suppressor. I spoke to John Noveske about 8 years ago on the phone about the KX3 when I called to order one. He explained to me about the pressure increases then and how it increased reliability in some guns because of additional pressures.

yes I agree, the whole reason of the pig was to increase pressure for short barrels. and its a lot like a suppressor because the noise all goes forward and its friendly to the shooter, noise friendly.

I still have my pig and its now on my 15-22, the wife does have her 15-22 suppressed and the pig splits the difference between her suppressed gun and a unsuppressed 22.

plus everyone thinks its a suppressor on my 15-22.  :)

just be clear its not a suppresser. 

[Guest sL1k]

yes I agree, the whole reason of the pig was to increase pressure for short barrels. and its a lot like a suppressor because the noise all goes forward and its friendly to the shooter, noise friendly.
I still have my pig and its now on my 15-22, the wife does have her 15-22 suppressed and the pig splits the difference between her suppressed gun and a unsuppressed 22.
plus everyone thinks its a suppressor on my 15-22. :)
just be clear its not a suppresser.

I never thought about putting one on my 15-22 I like the way you think.