New Magpul UBR-2 bolt not locking back on PMAGS

[M4Fundi]

I have emailed Magpul and will call later today. Love Magpul, but would love to hear any ideas here to for sake of knowledge...

Rifle functioned perfectly with ALL mag types before install.

Build is Noveske Gen 3 Lower, VLTOR MUR-1A upper, Noveske CL 16" Mid barrel, Geissele LPK, Geissele Trig, Noveske BCG, VLTOR A5-H2 RE... runs perfect!

Added UBR-2 with A5-H2 system and setup without tube spacer as directions advise.

All GI mags GTG, ALL PMags fail to lock back.

I do not believe this is a PMAG problem

A. I'm guessing either the UBR's tube is different length and is causing this...
B. Is this a tube problem or is this gonna be a buffer weight problem? or?
C. Is this causing an over or under gassed situation/over sprung or undersprung?

[MistWolf]

Does the bolt lock back when the action is cycled manually?

[10MMGary]

Last edited by M4Fundi; Today at 10:40. Reason: Im a dumbass

Okay spill the beans on what was actually the reason/cause of the failure to lock back.(so I don't become the dumbass,,,,,again)

[M4Fundi]

The BCG locks back perfectly manually on both GI & Pmags... i swapped BCG , CH, complete upper, lowers and manually all is GTG.

Only when firing with Pmags does it not lock back.

VLTOR has me taking measurements next week and

Magpul says that it MUST be a gas issue and that either the bolt gas rings or poor ammo is to blame

Ill check next week when i get back

[M4Fundi]

I dont believe its gas rings or ammo

[MistWolf]

Did you change your buffer or spring when you switched?

[M4Fundi]

I just got a couple other ARs and swapped components around to make sure it wasnt a bolt catch, CH, BCG, etc... this was all done manyally not shootin... when i get back in tow i will check rings and ammo and (VLTOR asked me to take measurements of the UBR tube) and then we will know if its a UBR issue.

Kudos to Magpul an VLTOR cust-service they have both been very quick to reply and offer assistance... far better than most gun product companies!

[BrigandTwoFour]

After reading this, I checked my newly installed UBR 2 rifle as well. I haven't had it out to the range yet, but low and behold I could not get it to lock back when manually cycling on one of my 20 rd PMAGs (earlier generation). That one works fine on the others. The bolt catch seemed to work fine on my other M2 PMAG 30s, so it might just be that one magazine.

Still, I found it odd. If I have time, I'll try swapping that lower on to other uppers and see if it makes a difference.

[MistWolf]

I have a couple of MagPul 20 rounders that my AR won't lock back with

[nightchief]

Have shot my UBR2 equipped rifle (18" mid length) now several times, with A5H2 and green spring, and I use Magpul G3 20 rounders almost exclusively. No issue with lock back. Sample of one.

I too had a couple of the older Magpul 20 rd mags and they were a problem with locking back.