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

[rockchalk06]

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

 

[GC7]

Undersized gas port on barrel?

 

[rockchalk06]

Undersized gas port on barrel?

God I hope not BA/Aero 18" barrel purchased new. It "looks" normal. I'd have to mic it

 

[KurtM]

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.

 

[kingfish]

How much distance do you have when pulling the charging handle from the time the bolt lockback engages till the handle is fully retracted?

 

[Oldman62]

I agree with the undersize gas port hole in barrel.

 

[JEVapa]

So without the spacer, I take it that it's the same as an AR10/SR25 carbine extension? With the spacer, make it the same as an AR15?

Try removing the spacer, put a carbine spring and a regular carbine buffer (H2), and see...kinda what @KurtM said.
Keep in mind, that the little AR308 (DPMS) buffer should use a shorter spring; this is to make up for the longer carrier...the standard carbine spring can stack up - unless you go with an AR10 extension and standard spring/carbine buffer setup.

 

[rockchalk06]

How much distance do you have when pulling the charging handle from the time the bolt lockback engages till the handle is fully retracted?

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

 

[JEVapa]

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.

 

[rockchalk06]

So without the spacer, I take it that it's the same as an AR10/SR25 carbine extension? With the spacer, make it the same as an AR15?

Try removing the spacer, put a carbine spring and a regular carbine buffer (H2), and see...kinda what @KurtM said.
Keep in mind, that the little AR308 (DPMS) buffer should use a shorter spring; this is to make up for the longer carrier...the standard carbine spring can stack up - unless you go with an AR10 extension and standard spring/carbine buffer setup.

It's supposed to be an A5 length tube