***Fixed!***What AR-10 Buffer/Spring are you running with the Magpul UBR?

[rockchalk06]

I would not do that.

1 will it cycle and lock back with 1 round in the mag?

No.

2 have you tried different types and brands of FACTORY ammo?

Yes.

3 do you have the correct size gas port?

It's .089” what's listed on Aero's site

4 check the gas "shadow"(carbon residue) around the gas port at the gas block(remove it), does it sit centered.

No leakage marks. It's centered

5 Is the gas tube correct legth?

Yes. Rifle length

6a do you have the correct buffer and spring?

I don't think what I have is what's needed for the A5 tube. H2 and new Sping on the way

6b does the BCG travel the correct amount? not too far, but far enough.

Yes.

6c is the buffer/spring combo allowing the bcg to travel far enough?

Moves a little over 1/2” after it pops the mag catch up

 

[Seadog]

I did sir.

Pict is attached
View attachment 328239

That looks good

 

[rockchalk06]

Problem solved. So after talking with @JEVapa, people at Tubbs, Aero and Magpul, my problem was my buffer tube.

I think I have this terminology correct, but the UBR uses an A5 system. 7 3/4” deep, longer than a Carbine and shorter than a Rifle.

Everyone recommend the Tubbs 308AR flat wire spring. Also the H2 Carbine Buffer. Magpul guy said he's running an H3 to slow the dwell time even more, but that's for a later date.

I found a Tubbs 308AR spring here locally (last one I think) and an H2 Carbine Buffer.

Fingers crossed, loaded 5 mags with one round each of 2500ish FPS 175 BTHP. Gas port fully open, each locked back perfectly. Next, I load 5 rounds in two mags. Fired each mag from bolt lock and it locked back both times and cycled perfect. Lastly I loaded one round, topped the mag at 20 rounds and fired. Runs as it should now.

Soft shooter too. Very soft. I did try and tune the gas block some, got down to 3 positions in comfortably, but I'll leave it open for now until my R2 gets here.

Thanks again for all the help OSA!