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

[JEVapa]

With the spacer, almost 1/2" without, maybe 3/4"?

That is just about the difference between a 308 carrier with a stacked spring and an ar15 carrier

 

[GC7]

Possible way to check gas port size without taking apart rifle:

If you can find fine wire in different diameters (too big, too small, just right) you can bend the end into an L shape (only about 1/8" long so that you can fit into the barrel) and then "gauge" the hole from the muzzle end.

Related pic to explain what I mean:

And maybe you even have an obstruction in the gas port which this could help dislodge.

 

[rockchalk06]

One more from the bowels of ppt and word and google:

DPMS LR-308 Style Rifle
18"rifle0.308 0.0960
18"car0.308 0.0635
16"mid0.308 0.078
16"mid0.308 0.081
16"car0.308 0.07
16"car0.308 0.073
16"car0.308 0.076
20"rifle0.308 0.0935
20"rifle0.308 0.096

These are ish - your adjustable gas block only works up to the port size on your barrel.

Rifle Length tube and port measures .087"

 

[JEVapa]

Rifle Length tube and port measures .087"

is yours a rifle gas or midlength gas?

 

[Seadog]

Still have some cycling issues with my M5 build and I'm stumped.

I'm running the KM .308 Buffer. 2.75" Long @3.6oz and a .308 Carbine Spring. The UBR Gen 2 has its own tube which is a tad longer. They provide you with the spacer for this.

With and without the spacer, no go. Will not lock back on an empty mag. 6 different mags and ammo running 2400 FPS up to 2750 FPS, no go. Will only pick up the next round maybe 10% of the time.

Can pull the charging handle back and lock it on all mags no issues.

Gas block is all the way open too. I've ran it all the way closed to open (16 clicks). No carbon showing anywhere around the gas block

This what has me think buffer and spring?

Any help would be appreciated
Thanks OSA

Did you build this rifle? What does your gas tube look like in the upper receiver when you look at it?

When you’re looking at the piece of gas tube inside the receiver in that moon cam cut out, it should be in the middle of it.

If it’s shorter than half, then it may not have enough dwelltime, if it’s a little more, it should be ok.

 

[rockchalk06]

Did you build this rifle? What does your gas tube look like in the upper receiver when you look at it?

When you’re looking at the piece of gas tube inside the receiver in that moon cam cut out, it should be in the middle of it.

If it’s shorter than half, then it may not have enough dwelltime, if it’s a little more, it should be ok.

I did sir.

Pict is attached

 

[rockchalk06]

Tubbs AR308 Spring ordered along with an H2 Carbine Buffer. I'll try and run this combo without the spacer to see if it runs.

I did clip several coils off my spring, KM buffer, spacer and tried that today. First 5 rounds locked back on 5 different mags with the Gas Block fully opened up. I backed it down one to fine tune it and the 6th didn't lock back. Opened the Block back fully, and never locked the bolt back again. I even tried loading 2 rounds in the mag to see if it would pick it up, no go. Ejection is super weak too. Maybe 1' back and 2' to the right.

Spoke with @JEVapa (huge fregging help by the way brother) and my local shop. Both said they think the Tubbs and H2 will help, shop thinks its the UBR Tube.

 

[rockchalk06]

Also, I thought off a whim that maybe the first 5 rounds somehow knocked my Gas Block out of alignment. Double checked torque, still at 25" pounds and my marks are still lined up. No gas leak signs around the block either.

 

[rockchalk06]

Cool video on the Tubb Spring

 

[mouthpiece]

I am in doubt that it is a buffer, spring issue, but the quickest way to tell would be to take a carbine spring and buffer out of a functioning 223 carbine and drop it in and see. It won't hurt the 308 for a few rounds and it would tell you about the spring/buffer you have. Just make sure the buffers are the same length or you have spacers to make up the difference.

I would not do that.

1 will it cycle and lock back with 1 round in the mag?
2 have you tried different types and brands of FACTORY ammo?
3 do you have the correct size gas port?
4 check the gas "shadow"(carbon residue) around the gas port at the gas block(remove it), does it sit centered.
5 Is the gas tube correct legth?
6a do you have the correct buffer and spring?
6b does the BCG travel the correct amount? not too far, but far enough.
6c is the buffer/spring combo allowing the bcg to travel far enough?