AR bolt not holding open after UBR stock installation. thoughts?

the_klenzer

the_klenzer

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Hey guys! Picked up a UBR stock for my AR off EE and installed it a couple days ago. Went to install a BAD lever tonight and realized that the bolt will no longer lock back in place (with or without the BAD). The bolt locks, but only ever so slightly, and literally if I give the rifle a hard shake, it'll snap closed. After doing that (ok mostly for fun) a few times, now it will barely catch at all.
I had it out at the range two weeks ago and shot it with no bolt hold open issues, so I'm assuming this is directly related to the UBR install. Using a carbine length buffer/spring that I had on the same AR with a previous stock, though the UBR of course has it's own tube. Maybe just a tiny bit less space in this tube is causing more force on the spring than the hold-open can keep in check? That's the only thing I can think of, just wanted to look for some outside advice before I go cutting the spring.

Thanks!
 
Ran a UBR on my Stag for a couple of years with a carbine buffer without any issues, I would take a closer look at your bolt catch see if it is worn out.
 
The UBR buffer tube fits in to the same depth as my previous one, I was checking it during the install. The bolt definitely pulls further back past the lock point by 1/8 or more, as I can feel it sliding back into lock as I release the charging handle while pushing in on the bolt hold button. So something like... pull all the way back, push button in, bolt slides a tiny bit forward, catches and the slips out and snaps closed with pretty much any rifle movement, or sometimes on its own.
 
With the upper and lower opened up, does the bolt catch look damaged at all? The spring and the spring pin for the bolt catch move freely? (AKA the bolt catch moves without any grinding)

With the lower by itself, does an empty magazine lock the bolt catch up, or does it look like it's barely moving the bolt catch at all?

A picture of the back-side of your bolt catch would be good!


The Magpul UBR buffer tube internally should be identical to the standard carbine buffer tube, so it's hard to imagine that they didn't cut the buffer tube hole deep enough. And it sounds like your bolt does indeed have enough rear travel.


You could also check your gas rings in your bolt, if they're lining up it MAY cause just enough pressure loss to prevent it from locking back properly on the last round.
 
The UBR uses an entry length buffer tube. It screws it and stops at a certain position. Not much can go wrong with it, and there no real way to screw it up. It's a more idiot proof system then a typical carbine tube.

It sounds liek something to do with the bolt catch. It's common on my machines that the bolt will slip closed if I bump the rifle.

Now the question on my mind is, will it stay locked with the mag in. It should not slip off with the mag in place holding the lock up.

Meanwhile, BAD levels have been known to cause these kinds of issues.
 
It almost sounds as if the bolt catch is hooking on the front of the bolt carrier instead of the bolt itself. If this is done the bolt appears to be locked back, but can sometimes fly forward if bumped. Are you able to pull the bolt all the way to the rear?

Perhaps the buffer tube is screwed in too far like others have said?
 
F22_RaptoR said:
You could also check your gas rings in your bolt, if they're lining up it MAY cause just enough pressure loss to prevent it from locking back properly on the last round.
Click to expand...

Fun fact: I've shot an AR with only one gas ring to "test" this kind of myth. The AR ran alright. This shouldn't be a problem.
 
Awesome feedback, thanks! ... I'll run these various options, take pics and have updates tonight.

I have another AR that I can swap parts from to narrow it down as well, will give that a run too.
Thanks again!
 
-[Crosshair]- said:
Fun fact: I've shot an AR with only one gas ring to "test" this kind of myth. The AR ran alright. This shouldn't be a problem.
Click to expand...

You are correct, it shouldn't affect it at all, but if you're using light loads, it may be just enough to stop it from reliably locking back (1 or 2% less gas pressure). The gun should theoretically run just fine (aside from not going all the way back enough to catch the bolt stop) but another thing to eliminate as a problem.
 
Ok, updates...
Pulled my other AR out.
1) Swapped the buffer springs, the UBR AR still did not lock, the other one locked fine (noticed that one of my buffer springs is a solid couple rings shorter). Then swapped back. So not the spring.
2) Swapped the BCG's between rifles. The UBR still didn't lock back, the other one locked. So not the BCG.
3) Swapped the UBR out and my A2 stock back in (with the A2 spring/buffer of course!) and presto, the AR is back to locking perfectly!
This leads me to believe it isn't the mechanism, but must be some combination of the UBR specific buffer and spring length. I put the UBR specific buffer side by side with the carbine buffer tube from my other rifle and it is definitely shorter, probably by 1/2" (I should have taken a photo).

So, the only issue I can think of is too much spring for the locking mechanism (either pressure or volume) and the only solution is to start cutting it back a ring at a time until it locks. I think it must be related to the spring pressure, as it seats in to place, but snaps closed quickly.

Does this jive with you guys?

If so, I'll start cutting tomorrow night. I will of course have to buy a backup spring just in case I botch it. :)
 
Is the screw that goes in from the rear too long? Try backing it off, see if the bolt carrier group will go further to the rear.
 
Update:
I just tried it without the screw at all, and no dice. Tried my other working AR again and realized the charging handle felt different. I pulled it all the way back, drew some lines on the charging handle in sharpie, and then put my A2 buffer tube/spring back on it and... sure enough, with the A2 setup it pulls back at least 3/8" if not a full 1/2" more. So with the UBR when it was half locking, it was actually catching on a different part of the bolt as it was almost a full 1/2" too short.

So, conclusion must be that my carbine spring is way too long (remember I tried two different ones) so a "normal" carbine spring may not work with the UBR. Since the UBR tube is specific to that stock, I'm guessing that means the only solution is shorten the spring. Did some Googling and the word is it needs a 10.5" length spring, or a 29 coil spring - depending on who you ask. So looks like next step is to cut it.

I also compared the UBR tube to the standard buffer tube length on the other rifle, and the UBR tube is a solid 1/2" shorter. Will update once I cut this up and see if it locks.

Edit... measured it... Only 10" but 36/37 coils. Does this jive with a normal carbine buffer spring?
 
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Alright so.
I'm measuring my UBR setup for you to compare.
Also, if you look carefully at the back of your upper where the charge handle enters, you can see the back of your gas key when it's pulled all the way back.

I use a WolffXP spring with a Slash SS A/T 6.0oz buffer.

The spring is: 11.3/8" Long. 36 coils.
With the buffer, about 12".

I'd say your spring is fine.
Or actually, according to my info on file, it's under spec and it probably due for replacement so don't cut it.
"Relaxed Length: 11.250" Minimum: 10.1/8"

I removed the stock but did not unscrew the tube. The tube seems to screw in flush to the inside of the lower.
I don't have a standard carbine stock to compare it to.

The UBR tube is: 7.1/4" from the inside of the lower to the rear shoulder.
7.7/16" from the inside of the lower right to the end of the tube where the screw goes in.

The UBR screw is: 5/8"

A standard Carbine buffer I have on file is: 3.1/4"
The buffer body is: 2.13/16"
The buffer rubber end cap is: 3/8"

Silly question, but did you check to see if anything is jammed inside of it.
It's not common, but possible.
I use a 308 buffer in my AR-15 Rifle stock. The 308 rifle buffer is shorter then the 5.56 version so I slipped $2 worth of quarters wrapped in hockey tape in back the tube to make up the difference.
 
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