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AR Pistol Not Locking Back After Last Round
Posted on 12/9/20 at 1:19 am
Posted on 12/9/20 at 1:19 am
Just built an 8.5" pistol chambered in 5.56 using a kit from PSA with an FN15 stripped lower
It cycles just fine but the the bolt carrier doesn't lock back on the last round
Kit came with a standard carbine buffer and spring so it's not about an extra weighted buffer or stiffer spring
i use standard 30 round magpul pmags
I fully lubed it before shooting it the first time, shot about 50-60 rounds through it, tried multiple times with a single round mag to see if it would catch but it didn't
Is this an issue of break in? should i use a reduced power spring? could it be a gas block issue?
It cycles just fine but the the bolt carrier doesn't lock back on the last round
Kit came with a standard carbine buffer and spring so it's not about an extra weighted buffer or stiffer spring
i use standard 30 round magpul pmags
I fully lubed it before shooting it the first time, shot about 50-60 rounds through it, tried multiple times with a single round mag to see if it would catch but it didn't
Is this an issue of break in? should i use a reduced power spring? could it be a gas block issue?
This post was edited on 12/9/20 at 10:33 am
3
Posted on 12/9/20 at 1:29 am to DVinBR
The BCG has to travel all the way back in order for the catch to engage. On those ultra short barrel guns you are getting a ton less gas to drive the BCG. Those pistols are notoriously fickle and often have to be tuned to work.
First, cycle the bolt manually while applying pressure to the BOTTOM of the bolt catch. Does it engage at the rear of travel? If not, you may have an issue with your buffer tube and spring/buffer assembly. If it’s a short tube it may require a special set up to allow the full length of travel.
Next, cycle it manually with an empty mag in place. Does it lock back? If not, try another mag.
Lastly, you can’t use too much lube on an AR. Run it wet. Cue the guys who are going to come in here and tell you to run it dry for some reason.
First, cycle the bolt manually while applying pressure to the BOTTOM of the bolt catch. Does it engage at the rear of travel? If not, you may have an issue with your buffer tube and spring/buffer assembly. If it’s a short tube it may require a special set up to allow the full length of travel.
Next, cycle it manually with an empty mag in place. Does it lock back? If not, try another mag.
Lastly, you can’t use too much lube on an AR. Run it wet. Cue the guys who are going to come in here and tell you to run it dry for some reason.
Posted on 12/9/20 at 5:32 am to jbgleason
Agree with jb. Not enough gas, bad mag, or once I had a stock blocking the bolt from traveling far enough to lock by a few mm. It would lock back manually over and over but not while firing single/last round. It was a rifle length fixed Luther ar stock. I moved the cheek riser back one detent and problem solved.
Posted on 12/9/20 at 7:04 am to DVinBR
I built an AR 9 with PSA lower and it does the same thing. It said in description though that most pistols won’t have BCG stay back after last round.
Posted on 12/9/20 at 7:06 am to jbgleason
JB nailed it. I would recommend an adjustable gas block. That should fix your issue.
Posted on 12/9/20 at 8:22 am to jdavid1
quote:
JB nailed it. I would recommend an adjustable gas block. That should fix your issue.
Not if it's not getting enough gas. The choke point is the gas port in the barrel, not the gas block. An adjustable will let you dial back an overgassed gun but it's not going to help for undergassed.
Assuming everything else is correct I'd try a lighter buffer. There may even be buffer springs designed for short barrel pistols, but I've never messed with barrels that short.
Posted on 12/9/20 at 8:44 am to Flats
quote:
An adjustable will let you dial back an overgassed gun but it's not going to help for undergassed.
I "had" a pistol that was basically a single shot.
This was a very inexpensive build and that barrel was an AR stoner from Midway. Not much invested in it.
Got my son to shoot it and I did a slow mo video of the gun to see what was going on. The BCG would only move back maybe an inch or so.
Did several things to try and remedy it but nothing helped. Finally took the barrel and drilled the gas port very slightly larger.
Has run perfectly since then
Posted on 12/9/20 at 8:50 am to bbvdd
quote:
The BCG would only move back maybe an inch or so.
Dang, what was the barrel length?
Posted on 12/9/20 at 9:00 am to Lsudx256
quote:
I built an AR 9 with PSA lower and it does the same thing. It said in description though that most pistols won’t have BCG stay back after last round.
Yeah, PSAs PX9mm lowers don't support last round hold open, their newer AR-V 9mm lowers do.
Posted on 12/9/20 at 9:03 am to Flats
quote:
Dang, what was the barrel length?
10.5"
Posted on 12/9/20 at 4:03 pm to DVinBR
Here’s your problem
....
J/k, I’d eventually like to build one too, but wouldn’t trust my life with my first one.
Good way to learn though.
quote:
Just built an 8.5" pistol
....
J/k, I’d eventually like to build one too, but wouldn’t trust my life with my first one.
Good way to learn though.
This post was edited on 12/9/20 at 4:04 pm
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