build parts list:

-SLR B30 ambi lower w/ambi bolt catch
-SLR B30 billet upper receiver
-SLR 14.78 ion hybrid mlok
-SLR sentry 7 titanium gas block
-SLR ambi mag release
-SLR low profile picatinny rail mlok
-SLR gas tube nitride


-Geissele Ultra Duty mag release
-Geissele SD-E Trigger
-Radian SD CH  
-Radian Talon ambi saftey
-Criterion 16 inch chf cl mid length gas barrel  
-Magpul ubr gen 2
-slash heavy xh buffer & spring
-Aero NiB BCG
-Surefire Warcomp
-atlas bipod  
-Strike Industries take down pins
-KNS precision firing pins


first time taking it out, and it wont extract a 2nd round from the magazine (used magpul pmags and lancer mags) , I tried adjusting the gas block, when I almost closed the gas, it fired one and did not even want to eject it

when I opened it up, it would eject the first round but not extract a second from the magazine (bolt would close without a round taken)

I opened up the gas and eventually got 4-5 rounds off in a row, and was never able to duplicate that.

pretty much consistently would not pick up a second round from the magazine

fmj brass ammo normal stuff.

either gas issue but leaning on buffer + buffer spring / bcg

How large is the gas port diameter? I had to open mine up to .086 from .075 to get it to cycle reliably. The fact that functionality improved after fully opening the GB suggests you are probably undergassed.

I am running one of Slash’s 5.5 oz buffers and spring FWIW

reliability never really improved, I was not able to recreate the success, and the same problem persisted. I was able to eject rounds fine.

I would have to check on the gas port diameter ?? I have an adjustable gas block on there

under gassed..... my best guess....

not cycling far enough to pick up the round. Try some hotter ammo and see if it gets better. other than that, check the gas port size, your adjustable gas block can only adjust what the gas hole in the barrel feeds it..... cant get more if the gas port wont feed it more.....

will probably bring it in to my gunsmith, called him on the phone and because I walked it through its paces on the adjustable gas block and noticed no change, he is inclined to believe the buffer is too heavy and not going far enough back to pick up a round.

we shall see!! either gas or something to do with the buffer/spring-bcg

I pretty much only have fmj brass 147 grain type ammo, If the gun cant feed normal ammo, I would have no need for it. I dont buy guns that are finicky, they must work

Would it lock back on an empty mag?  That is how you should be checking for appropriate gas adjustment when tuning an AGB.  If you didn’t check this, hit the range again and test.

On its face, it sounds like an undergassing issue.  I’d swap to a standard carbine weight buffer before opening up the gas port or anything permanent like that.  It sounds like you’re very close to properly functioning, so it may be enough to get you over the edge.

ETA:  I see the UBR uses it’s own receiver extension.  Which Slash buffer are you using (shorty 308 or standard carbine length)?  Are you able to lock bolt back by hand?  Is there any damage to the rear of the lower (from the gas key/bolt carrier impacting it)?  Are you using the “buffer spacer” noted in Magpul’s literature for the stock?

ETA2:  I have that same barrel.  It’s nowhere near being undergassed.  I’m leaning a little more towards a buffer/receiver extension compatibility issue.

I never got the opportunity to get to an empty mag.

I would have the bolt back, insert a loaded magazine, send the bcg forward loading a round, fire that single round, and the bcg never picked up another round.

I tweaked the gas system closed to pretty open, somewhere along opening it up, I had 4-5 shots go off in a row perfect, i took a break, came back and it did the same thing it always did.

I believe I installed the UBR correctly reading the manual.

I initially talked my build specs over with clint and slash heavy buffer and he recommended this - XH Carbine Buffer (8.5 oz) w/.308R Buffer Spring - so I went with that, we will see what my gunsmith says, if that buffer is correct or not. I believe it will be a simple fix from process of elimination.

It is at my Local gun smith currently and he is going over it. He believes it is a buffer / spring issue because of the BCG not picking up a round. We will see.

Only thing I intended to do with this build is get a JP low mass BCG instead of the Aero NiB

^^^

Pro-tip:  load one round in the magazine.


In all seriousness, in for answers as I have a 308 build in process.

Also, the Criterion barrel is trash. Send it to me for proper disposal

haha yea no joke right? on all accounts! haha

Well, if you could lock it back by hand, then it’s not an issue with too long of a buffer.

For future reference, it’s best to tune an AGB by loading one round in a magazine and testing to see if it’ll lock back.  If it’ll lock back, the carrier is traveling far enough rearward to function correctly.

Interested to see what he figures out.  If it truly is a gas issue, I’d first verify correct alignment of the gas block with the gas port, then try a lighter buffer before drilling out the gas port.

Manually cycle it.  If it strips in the first round, ejects the live manually cycled one fine, and then manually feeds the second round - then the issue isn't likely the magazine or parts dimensions comparability.

Reason I mention this, is AR10s are a whole other animal.  And my first one would not feed, because the magazine latch was too low, so bolt wasnt engaging the back of the rounds.  Even when I manually cycled.  Had to warranty replace the whole lower.

well that would be embarrassing to a company like SLR, with the cost and reputation they have. It was sent out with a faulty ambi mag catch that had the be replaced, maybe its a lower spec. We will find out within in the next few days to week I expect

it was over gassed, adjustable gas block had to be tweaked

guess my first time using it, didint take enough time trouble shooting, was probably turning the gas too much

I second the suggestion to load one round and fire it and see if it locks back on an empty mag.  If not I would replace the buffer with a much lighter one.  Your spring might be too strong too.  Usually a AR 10 barrel is over gassed. Your ejection pattern on your fired brass should be between 3-4 o'clock for best results from tuning your gas system.