Help me troubleshoot my new .223/5.56 AR pistol build...

DarrenC

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It's not cycling properly. Every round ejects perfectly but it doesn't always pick up the next round.
Sometimes I can get through 5 rounds fine then either the bolt closes without a round or I get this half strip/load with bolt marks on the brass. You can see the round kind of pointing up and in but the bolt is wedged closed on it. Sorry for the crappy pic sun was bright and phone wouldn't focus inside.

I think the bolt just isn't getting all the way rearward to properly strip and grab the next round. Buffer or buffer spring?

This is a new build. Generously lubed (not excessively) and everything manually cycles fine with no catches or rubs, including buffer and buffer spring. On disassembly I see no problem areas either. I've put 150 painful rounds through it now waiting for it to loosen up and it isn't. I've tried many different mags with various rounds loaded with no difference. Mags all work in other ARs.

It's a this kit with this lower build kit. I built it as shown and had the cycling issues I described above. My SBA4 came in and I switched it out and getting same result. Didn't expect it to affect it but nonetheless...

Some quick measurements, buffer tube is ~7 1/4", spring is ~10 5/8" but I don't know the weight.

Any ideas? Replace the spring? I wonder what mine is now and how low to go.
 

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If you fire a single rd. Does the bolt hold open function? If no, its not going far back enough..so maybe enough to clear a spent case, but not enough to allow fresh rd to be stripped or bolt hold open to function.

Hows gas system look? What kinda gas block? Is it placed correctly? Tube have any extra holes? Gas rings on bolt..they staggered?

Also what kinda ammo? Is it low powered 223? Does 556 ball function properly?
 
Do you remember where was the brass landing as it was ejected? Was is just kinda being spit out or being firmly ejected at least 4 plus feet to your right and hopefully a little forward.
 
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Under gassed is what I’d say. Check all your alignment on the gas system


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What @Jabroni said.

You can also try the buffer and/or spring from another carbine to help identify the failure. Same with bolt, or entire BCG if that’s easier.
 
If you fire a single rd. Does the bolt hold open function? If no, its not going far back enough..so maybe enough to clear a spent case, but not enough to allow fresh rd to be stripped or bolt hold open to function.

Hows gas system look? What kinda gas block? Is it placed correctly? Tube have any extra holes? Gas rings on bolt..they staggered?

Also what kinda ammo? Is it low powered 223? Does 556 ball function properly?

It does not hold open which also made me think about the spring/buffer. I wasn't too concerned about gas since it's a new gun. I inspected the gas system and all looks good. Checked for any gas blockages and it's clear. Alignment is good.

How do I increase the gas on a non-adjustable system? Tear it apart and booger with the barrel hole?

I didn't think about switching out the BCG....

Does the same with any ammo (.223 or 5.56).

Brass was back 45 deg and 5-6' out. Good ejects every time.
 
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You might be over gassed and cycling too fast to pick up a round. Ive seen it happen especially with 9mm builds. Try a rifle spring or a few ounce heavier buffer. .
 
I would save opening up the gas port for a last resort. Or you could send it back to PSA and let them deal with it but I would be concerned with how long that might take.

Try swapping a few parts, one at a time first. It may be a really simple fix.
 
If you have another bcg, worth a shot swapping. If not id just check gas rings and make sure theyre staggered as mentioned before..

If you opt for a larger gas hole you might as well drop on an adjustable block too.

Havent run into buffer issue on a 223 pistol. I think mine is an h2, but dont quote me, runs like a boss though. Did need to add weight to my 9mm build but thats 9mm..lol.
 
It does not hold open which also made me think about the spring/buffer. I wasn't too concerned about gas since it's a new gun. I inspected the gas system and all looks good. Checked for any gas blockages and it's clear. Alignment is good.

How do I increase the gas on a non-adjustable system? Tear it apart and booger with the barrel hole?

I didn't think about switching out the BCG....

Does the same with any ammo (.223 or 5.56).

Brass was back 45 deg and 5-6' out. Good ejects every time.

Big assumption. It's a cheap barrel/block. It's not uncommon to find one with an out of spec gas port. Could be slightly off center or too small, or there could be a flash of machining partially blocking the hole in the barrel or the block. I've seen all three. How do you know alignment is good? Did you knock out the taper pins and mic the hole and observe a carbon ring around the port?
 
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You might be over gassed and cycling too fast to pick up a round.

I have seen this, too. More than once actually.

Where is the brass ejecting? If it is forward of the ejection port and the gun feels a little violent, this could be the problem. A slightly big gas port on a carbine gas system can be very high pressure and can be too fast to strip a round. So, something to be considered. If that is the case an H3 buffer may be what you need to slow it down a bit.

In my limited experience, watching ejection tells you almost everything you need to know most of the time.

Edit: read that it is 45 degrees behind. That could be low gas as mentioned or case hitting of front of port and being kicked backwards. Something to consider and carefully look at next range trip.
 
Sorry for the delay guys but I finally got back to the range yesterday.

I pulled the BCG, buffer, and buffer spring from another AR-15 and stuck it in the bag to change out if needed.

I yanked the gas block off, manually (by hand, no drill) spun a bit in the gas port to make sure it was entirely open, realigned and reinstalled the gas block. I *THINK* the gas block was ever so slightly off-center. The gas ring burn on the barrel where the block sits on top of the port was not centered (port was off to one side of the burn). I centered it, reinstalled the block, and shot 100 rounds through without issue.

So, it all boiled down to a slightly cocked gas block. I'm pretty amazed that just that much misalignment mattered. The port was still fully within coverage of the block, just not entirely centered. Needed to put enough rounds through it for the exhaust to outline the block coverage and voila!
 
The 10.5in upper I have like that is somewhat over gassed, but runs well... I use a H2 buffer and a sprinco buffer spring in my pistol lower to tone it down some. Still runs whatever ammo fed to it well, but isnt quite so violent.

It is interesting how much effect a slightly off center gas block can have!

I hope it runs well for you!
 
The very first thing I do on a 'budget' upper is pull the rail off and sharpie mark the centerline of the barrel port and the gas block port and make sure they're aligned. Then I put
Hi-Temp Loctite on the setscrews and torque them to 30 in-lbs. That's just part of it for me. I also tend to true the reciever face, and bed the upper to the barrel extension with 609.
Haven't had major operation or accuracy issues with an upper in a long time doing that.
 
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