ar15 carbine, not unlocking

[jacUSMC]

Ok so I have a Palmetto State armory upper, carbine, 16" barrel, ATI poly lower. this info I think is inconsequential, but there it is. It is factory built, never or rarely fired...probably not more than a few rounds, why? becase it is a single shot AR at this point. Now I have considered the gas problems and buffer spring ect. but here is the deal.
You fire, the bolt may come back far enough to eject, or it may not, It will not come back far enough to chamber the next round. now there have been no FTF's or stove pipes, double feeds or anything like that. just charge it and fire again. I have dissasembled and cleaned every thing and it does the same. tried multiple loads.
I noticed while cycling though a mag manualy that the bolt/carrier would get stuck about an 1/8 of an inch back in the cycle and i would have to pull very hard or butt strike to get it to move out of lock. I examined the bolt and observed what appeared to be minor burrs around locking lugs, 800 grit finishing paper to smooth them out. ok long story short, I have been over the entire bolt and carrier, effectivly doing a manual brake in.
But literaly opperating the bolt by hand with the upper removed it locks and unlocks pretty well, but add a round into the chamber and the bolt will "catch"half way in its unlocking cycle. and I have to pry it unlocked with a brass cleaning rod. so with that book of a story, any clue to what may be going on here?

 

[crazysccrmd]

How's the chamber look? Might be a burr catching the brass on extraction.

 

[Blade First]

Is the brass on your cartridge scratched when you get it removed from the chamber? The last time I saw this problem, the chamber itself was rough as a corn cob and was "gripping" the brass so tight that it wouldn't extract properly.

Are you getting "claw marks" on the rim of the cartridge because the extractor is having to work so hard to jerk the round out?

Are you absolutely sure your buffer/buffer spring/buffer tube are in spec?

 

[jacUSMC]

I'm not positive about the buffer spring type, and the chamber "looks" clean and polished..but..
I am going to attempt to use a different bolt carrier group from a known to function fine AR and see if it still does the same, alternately I am going to try mine in the other AR to eliminate or confirm any other possibilities.

 

[jacUSMC]

also, I can drop a live or fired round into the chamber, make sure it is seated and run a cleaning rod down the barrel and easily push the round out, no sticking..obviously it is dissassembled in case abyone was wondering...

 

[jacUSMC]

sorry keep remembering things as I read.. there are scratches on the case sides but these seem to coincide with the locking lugs in the chamber upon extraction, you know the one side of the case is being pressed outward by the ejector pin.

 

[carbine85]

You eliminated the Fail to feed so what you have is a Fail to extract.
1) Check the bolt extractor.
2) Check the gas key and see if it came loose (under gassed).
3) Check the receiver extension and make sure it didn't come loose.
4) You didn't say what the ammo was. Try different ammo.
5) Is the gas block canted and blocking the gas port?

 

[eldon519]

Why dont you just send it back to PSA?

 

[jacUSMC]

PSA only honors original purchaser, I am not.

let see..as far as the gas block it is an A2 front sight set up, I have used multiple brands of ammo. It will extract when when the bolt unlocks, the extractor is good, the ejector is awsome.
I just returned from the range, and got it to fire 3 shots in a row..then single shot again. now, what are the chances that I have military bolt carrier group, and is there some kind of adjustment to that to ensure proper function?
Also the bols seems to have a hard time making the turn to unlock as I pull the carrier back by hand.
I did not build this AR, I figured the upper was factory made and so was the lower, and that the bcg should work like normal, but perhaps I was lied to by the fellow I got it from, no suprise, so I'm kinda in the dark on some things here.

 

[jacUSMC]

As far as the bolt being mil or not, It is new, and looks like a mil spec but not a military part. I filed a few more things here and there and the carrier now moves freely in and out of battery with ammo, sticking is very reduced...now to re-shoot, and hopefully just break it in with use.

I must say though with a A2 front sight and a del ton fixed rear, she shoots groups at mechanical zero, point of aim point of impact, OohRah!

 

[crazysccrmd]

Try swapping just the bolt from another rifle. If that doesn't work, try just the carrier, etc. Changing one piece at a time and when it functions properly you will know what the cause was.

If nothing changes try swapping the complete lower, in the unlikely event that the hammer is riding too high and snagging on the bolt carrier or the buffer tube is crooked and preventing the bolt carrier from sliding smoothly back.

If you fire a single round (and it cycles properly) does the bolt lock back on the empty magazine? If it does locks open consistently your gas system should be good.

 

[carbine85]

What are you using for lubricant and how are you lubricating?

 

[Matt Dillon]

Are you using factory ammo, or reloads? Is the cartridge OAL correct? Have you tried a different mag?

 

[jacUSMC]

lubing with CLP, normal stuff like I used in the Marines, we are not afraid to lube it...oh yeah..semper lubitupis...
factory and reloads, brass and steel cased. OAL is good as well as charge in the reloads. factory was new 3 different brands. etc.
I may have a fix, but I need to go shoot it again...

 

[barnbwt]

What were you filing that fixed it? I was sort of wondering if it was a pin/channel thing causing borderline misalignment when the bolt goes to unlock; could be binding the bolt lugs against the extension lugs enough on the back stroke to sap all your BCG momentum. Also check the gas port, both for alignment and blockage.

TCB

 

[gunnutery]

It's probably a rare occurrence, but you might as well check. Fall of last year I noticed the buffer on one of our department rifles. The pin that holds the buffer and rubber pad together kept backing out. I'd noticed it a few months before but we put it back in place. When I rechecked it, it was backed out again, just from riding in the car, it had not been fired at all.

I was fairly new in the maintenance business of ARs so I posed the question here on THR about the problem. Very few had ever seen it happen, but the general consensus was that if it came out far enough, it could hang up on the buffer spring and cause stoppages.

Like I said, probably rare enough that it's not your issue, but never hurts to look.

 

[jacUSMC]

I'm using auto body 600 grit paper, and so far Ive buffed...
bolt lugs, extention lugs, the bolt body, bolt face, the inside of the carrier, and finally the channel the cam pin rides in, and the last seem to be the thing that eased the most tension while manually operating the bolt. just need to shoot again...Oh I have checked the gas system multiple times, no obvious issue.

 

[dprice3844444]

ref buffer pin.if it's a roll pin,take it out ,go to hardware store and replace with a stainless one,and put put some lock tight on it.the hole may have been drilled oversize,or pin was undersized

 

[barnbwt]

"and finally the channel the cam pin rides in, and the last seem to be the thing that eased the most tension while manually operating the bolt"

Sounds like the bolt carrier slot may have had a burr or out of spec dimension that just barely cause the lugs to misalign on the back stroke. I thought to mention that because of the troubleshooting tail I read on another forum, wherein an AR10 bolt and barrel/extension were used to turn a G3 into a locked breech gas-op rifle. The trouble was the bolt carrier was home made, as was the cam slot, and it was just enough off that when under gas pressure, it would over-rotate the bolt head into the barrel extension as it tried to slide back, and bind up everything to the point a hammer was required to open it. Welding up and remilling the slot with a little extra care cured everything. Sounds like yours had the opposite problem, if removing a tiny bit of material helped (that's always easier than adding it )

TCB

 

[Ramone]

While you are at it, cheack the length of the Action Spring (aka Buffer Spring)

RIFLE: 11 3/4 Inches (29 85 cm)
minimum to 13 1./2 inches (34 29 cm)
maximum

CARBINE: 10 1/16 inches (25.56 cm)
minimum to 11 1/4 inches (28.58 cm)
maximum

 

[cfullgraf]

Try a lighter buffer if you are not already at the lightest one. For a carbine, a number should be stamped in the buffer. H0 is the lightest, H3 is the heaviest.

 

[breakingcontact]

How is the gas block aligned with the gas ports? I imagine they were drilled correctly but perhaps misaligned so it isnt getting enough gas?

 

[smovlov]

Check the rings on the back of the bolt. Make sure they are not aligned. There should be three of them.

 

[jacUSMC]

finally.

finally got to the range today. took a known good AR carbine, and started a parts swap, mine was still not functioning, and we have narrowed it down to the bolt carrier itself....now..no idea why. the 2 bolt carriers we have ar not the same, mine is what I am calling a military type, the other is a "sporter" type, not sure what to call it, but deminsionaly they are identical except for the rear of the carrier where the mil spec has more metal and the other is cut back, must have something to do with the way an M16 functions, no need for it if you ask me, Jarheads like to to shoot semi-auto anyway(unless its belt fed). anyway. so all the swapping back and forth the only thing that was consistent was the carrier was the same at every malfunction.
new thoughts, I may have missed a thread somewhere about this issue.
just so everyone understands..swaped bcg, buffer spring assy, charging handles, uppers, lowers, bolts, cam pins, etc. the bc is the malfunction, but, here it is, the bolt carrier will work in my friends AR( which is a 300acc) i know that throws off some of the varriables, but not much, :banghead:
there is anothe 556 ar we are gonna try, but who knows. everything on mine works with all my friends parts, except the bolt carrier itself. so Im gonna get another one, but does anyone know what the deal is?

 

[carbine85]

I went back through the threads and I didn't see where you checked some of the recommendations.
Your bolt carrier sounds like a full auto carrier and should be fine. Did you check to see if the gas key is tight and torqued properly?
Did you check the receiver extension and make sure it didn't come loose?
Do you have a Carbine length buffer & spring?
This might boil down to an out of spec lower and the mags not fitting properly or the hammer is slowing down the carrier.
I would reassemble the upper to original and test it on another lower. This should tell you which half the problem is in.