I have a 10.5 inch 5.56 PSA pistol.  It shoots reliably but does not lock back on the last round.  Its ejection pattern is close to the gun (drops the shells about 2-3 feet away max).  It came with a standard buffer but I replaced it with an H2 which did not help.  How do I diagnose over/undergassed and correct?

Sounds undergassed, short stroking. H2 would not help or make it worse, lighter buffer might help.

If the pistol buffer tube is shorter than a carbine length tube and you have a carbine length spring, it could be overcompressing the buffer spring, I would measure the buffer tube length by inserting a rod and measuring to marked depth, then measure a carbine buffer the same way.

Make sure the gas key is tight and staked. Go back to the carbine buffer. Are you using the proper buffer tube and spring? Are you using a rifle spring?

You shooting steel or brass cased ammo? Steel is under powered.

Going heavier is not going to remedy an under-gassed issue.  Pop out the pin in your standard buffer and empty the weights out of it.  If that fixes the problem you can add a mixture of steel and aluminum weights to tailor the buffer to match up with the port size.  Other option is to open the gas port a little at a time, but save that for last resort.

You didn't specify which PSA barrel (premium FN, etc.), but as a general rule, PSA short barrels are over-gassed if anything. Last I checked, FN was using a fairly excessive .080" gas port.

Which most likely means you have an issue somewhere in your gas system -- a leak, a misaligned gas block, a loose gas key, etc. I would start there, or go to the PSA forums and let them handle it.

make sure the gas blocks on right and check to make sure its also an ar15 buffer spring

Sounds undergassed. But check the gas rings just in case.

My assessment also. Misaligned gas block or gas key issue.

One of my 16" PSA rifles will NOT lock the bolt back on Wolf steel (Tula) ammo. What ammo are you using? (it should work with everything but some ammo is on the edge for gas pressure)

There is a video on youtube where they align all the gas rings and the rifle functions normally. They also remove the rings one by one and the rifle worked with only 1 ring. I think that for a properly set up rifle, the gas ring alignment should be a non-issue. But there is a HUGE variety among ammo: Independent primer-poppers .vs wimpy Tula. (I still make sure to stagger my ring gaps though just out of habit)

Sounds like too much spring for the gas its getting. You could use a lighter spring and shoot or spend a lot of time and money chasing a 'gas problem'.
Just realized none of us explained what is happening; the bolt is getting blown back far enough to pick up a fresh round on the way back home but not quite far enough to lock back on the empty mag. Its very close and only a slight adjustment is needed. First, put the original buffer back in it. Now with an empty mag locked, pull the bolt back and see that it manually locks back as expected - assuming it does, this shows that the spring is not stacking.
Now, as suggested by others, shoot some mil-spec factory ammo and see if it functions as-expected.

If the other suggestions don't work, remove the gas block and measure the gas port - you can use different size drill bits.  My last build with a 10.3" barrel had a .078 port and I used an adjustable gas block to tailor the ejection pattern to my ammo of choice.  If the gas port is smaller than .078, you might have to drill out your current gas port to a larger size.  Best to use a drill press, but a hand drill will work if you'er careful and go slowly.  Make sure to put a small wood dowel in the barrel to keep from drill bit from damaging the opposite side of the bore.

So, just ignore that there is very possibly a significant underlying issue that could affect reliability and try to band-aid it? Some basic tools and a few minutes spent checking the relatively few components of the gas system to determine the issue is hardly "a lot of time and money."



You're able to use an adjustable gas block because .078" is on the larger end for 10.3" gas port sizes. On the one hand, you're saying you're using an AGB to tone down the .078" port, then in the next sentence say you may need to drill out any port smaller than .078" -- seems a bit contradictory. An AGB can decrease the amount of gas, but not increase it.

In other words, there should be absolutely no reason you'd need to drill out your port just because it's smaller than .078". Personally, I'd consider .078" too large as it is -- although there are still manufacturers building/requesting specs of .080"+ *cough* FN barrels made for PSA/Noveske/etc. *cough*.

Not because it's required, but because it can help to mask other issues with manufacturer quality and/or allows out-of-the-box use of cheap, low-pressure steel case ammo like Tula that might otherwise need a break-in period or mild tuning to function with a smaller port -- most especially when low quality and low-pressure ammo are combined. That's assuming there is any real thought behind it at all; in many cases, it's simply dated thinking -- that short barrels need ridiculously oversized gas ports to function.  

The spec used by Crane/NSWC as well as Colt, DD, and FN (for govt contracts) is .070" for a 10.3" -- it's just about the perfect "jack of all trades" compromise size for suppressed and unsuppressed use. Now, if your port measures less than .070" and it's not for dedicated use with a suppressor, you might have a problem.

A quick search showed a lot of 10.5" barrels have gas ports larger than .070" (if you can believe what's on the internet)
KAK Industries 11" bbl with carbine gas is .084 - .086"
Mega Arms 10.5" bbl with carbine gas is .080"
Green Mountain 10.5" bbl carbine gas is .087"
Anderson 10.5" bbl carbine gas is .086"
Ballistic Advantage 11.5" bbl carbine gas is .073"
Ballistic Advantage 10.3" bbl carbine gas BA HANSON with low pro gas block pinned is not public (proprietary)

Not that many with .070" ports but there were a few.  I even found one reference to a PSA 10.5" barrel that measured .079", so an undersized gas port is probably not the OPs problem.

Are you shooting some wimpy ass ammo?
Try it with M193 or 855.

Also, heavy buffer will make short stroking worse, not better.

Yes, it's still fairly common.

As evidenced by your list, some manufacturers are still using utterly ridiculous port sizes -- there is absolutely no valid reason for a .086" or .087" gas port on a 10.3/10.5 barrel. Assuming it would even run suppressed reliably, it would be horrendously over-gassed. Prior to DD adopting the Crane spec .070" port, many had to resort to AGBs or combos like an H3 and Sprinco Red to tame the old .080-.082" port with a can. Goes to show just because a manufacturer chooses to do things a certain way doesn't necessarily mean that any real design or engineering thought went into that decision.

Hell, prior to the testing and development by Crane that selected .070" as the optimal "compromise" size for the Mk18/CQBR, Colt was using an utterly ridiculous .093" port -- based on Vietnam-era thinking that short barrels required massive gas ports to function. Luckily, DD and Colt eventually decided to evolve with the times. As far as manufacturers currently using a .070" port, that'd be Colt, DD, FN (govt contract SOCOM profile barrels), BA/Androcorp "Crane spec" nitride and BA "Classic Series" CL barrels, as well as LMT 10.5" barrels (0.071"). That's off the top of my head, so it's possible there might be one or two others I missed.

Don't get me wrong, I'm not saying those larger ports won't run or even that there isn't a place for 10.3/10.5 barrels with a port larger than .070" -- for a gun that'll primarily or exclusively be fired unsuppressed, with low-pressure steel case ammo, there is nothing wrong with a .078" ported barrel. When you're talking about budget barrels with what might generously be called "hit or miss" quality, it's probably even a good idea. Combine steel case ammo with rough and/or tight chambers and other quality issues, increasing the gas can be a requirement.

That said, there is really no valid reason a 10.3/10.5 barrel should need a port exceeding .080" under any circumstances. Otherwise, a .070" port is about perfect in my experience -- and most especially for anything that's going to see any amount of suppressed shooting. Personally, I won't buy anything larger than a .070" port regardless, but that's my own preference.

All of my 10.3 and 10.5" barrels have pinned A2 flash hiders. In order to get a good "Wrench-Bite" on the barrel-nut I always use a Brownell's 5 prong full circle barrel nut wrench. So the A2 Flash Hider's have to come off. I rest them in a Brownell's A2 Template, soak the taper pins overnight with PB Blaster several times... Support the barrel-extension end with a 3/4" piece of cherry flooring and whack the (2) Taper-Pins out with a big hammer the next morning. Every time afterwards I've taken an above decent set of digital calipers and inserted the backside of the calipers that protrude outward into the gas-port(s) 5-6 times and in a counterclockwise radius taking measurements each rotation. Below is what I've gotten for each.

Multiple Non-Crane-Spec'd 10.3" barrels from Ballistic Advantage/Andro-Inc. measured .078"-.079"...
One 10.5" barrel from MEGA  .080" to .081" consistently...
Two 10.3" "Crane-Spec" Barrels from BA/Andro-Inc measured out at .070-.071" consistently as well.

Every single set-up is ran with a JP "Enhanced-Carbine" spring and either a KAW-Valley H3 or a Colt H3 Buffer (both weigh between 5.3 & 5.4oz's), FA-Carrier, & the (same ammo regiment as well) is used in all... I.E. .55 Grain IMI, .55gr Privi-Partizan, and/or the similarly/Clone Loaded Federal/"Lake-City" XM 193 Ammo used for break-in and sighting-in any optics Secondly after the back-up And/Primary Irons are dialed in and out to 50 Yard Zero's each... (That is the range I like and of course  ....YMMV).

Every single 10.3" or 10.5"  Barreled Rig runs 100%, locks back on last round in either STANAG or MagPul mags; with a smooth recoil impulse, and either a 3:00 to 3:30 ejection pattern to about 4ft away and into nice little piles. Only a sample of (5 )I know... But all (5) have similar and very consistent operation, impulse, reliability, ejection pattern, and last round lock-back whether .070, .078, and .081 sized G-Ports with same aforementioned Recoil-Spring, Buffer-Weight, Carrier/Bolt, Barrel-Length, and Ammo.

The last .080"+ ported 10.5" barrel I had (Novekse medium profile CHF, made by FN) was so over-gassed that even an H3 and Spinco Red wasn't enough to offset the excessive gas and it was still exhibiting noticeably sharper recoil and forward ejection pattern at 1-2 o'clock (unsuppressed even) compared to .070" ported barrels with standard springs and an H2. I didn't even bother trying to run it suppressed as-is and pulled the FSB and installed a BRT .070" port reduction insert. After that, it ran fine, but I eventually sold it and replaced it with a Colt 10.3" .070".

I won't try to dispute your experience that you see no difference, but for me, the difference is clear and easily discernable. I've talked with numerous people with similar experiences -- just ask the cloners that ran DD barrels and the issues they had trying to suppress them with the old .080-.082" port.

Guy at the range this weekend was having trouble with his brand new PSA upper.  Set screws fell out of the gas block.

Not the first time I have heard of loose fasteners, and not just from PSA.  I have never bought an assembled upper from any maker, but if I did the first thing I would do is put a wrench on everything.

OP, are you running a BAD lever by any chance?

i just had 2 10.5 setups go tits up. and switched gas blocks to make them work. realigned everything and they run awesome