I just came into a new RR LAR 308 A4 Flatop 20" a few days ago. I have built many AR's and have all the tools, blocks and other stuff for the 223/556 rifles, but this is obviously a different animal in size. I have already ordered a 12" DD light rail for it, but had to get the barrel nut and bigger nut wrench from Daniel Defence directly as this rail kit was for a different 308 upper. My question on the stock is "can I use the UBR stock, and what do i need to change to make it work?" I'm looking for specifics regarding the extention tube, buffer and spring. If someone could point me to a specific source, i would greatly appreciate it. I know next to nothing as to what to use for the 308 buffer and spring, and want this rifle to run smooth and flawlessly. i have seen someone with a similar setup on this site, but I can't find the picture now. I have 2 other 223's set up this way with Talon arms tactitcal barrels and Acog scopes-they are sweet. But i will end up putting a scope in a Larue Mount for the 308. Any help would be appreciated. Thanks for your time.

-MP

UBR comes with all the parts except buffer and spring.  It includes a entry length receiver extension tube.  (which is the same style as a rifle tube, but shorter)

The big question is what stock was on the rifle before?   If it was a collapsible carbine stock, then just reuse your buffer and spring, and you are done.  Its far easier than messing with a barrel nut.   Just watch that buffer retaining pin which like to fly about the room.

If you had a rifle stock on before then things become more complicated.   Then your options are get a carbine sized buffer and spring from RRA or from Slash.

Edit:
I think all the 20" guns come with rifle stocks.  So you are going to need the RRA 308 sized buffer and spring.  I dont see any 308 parts on their website, so I would call them.   You need a 308 buffer, and the spring may or may not be the same–– so ask. (Its the same on Armalite 308, not on DPMS)

Rock River Arms, Inc.
1042 Cleveland Road, Colona, IL 61241
Tel: 309-792-5780, Fax: 309-792-5781

Or Slash ad:
http://ee.ar15.com/ItemView.aspx?iid=34371

It came with the standard A-2 Rifle Stock. I will give RRA a call and get what I need from them.
Thanks,
Mike

UPDATE-

I ordered a 308 carbine buffer and 14.5" spring from RR and installed it yesterday. The 308 buffer is too long for the Carrier to cycle far enough to strip a round from the magazine. I'm guessing that Rock River's extension tube for their carbine stock is just a little longer than the MagPul Extension tube. The spring would work just fine-35 coils compressed at .071 per coil is close to 2.5 inces. I took the spring out and just left the buffer in to make sure. Here are some pictures of how far back the carrier will go before the buffer bottoms out-

http://photos.imageevent.com/shadow40/myar/websize/IMG_2787.JPG

http://photos.imageevent.com/shadow40/myar/websize/IMG_2788.JPG


The buffer roughly measures 3.240. I'm sure if I cut the buffer's tip down about a 1/4", this would work and the carrier would cycle. I just called Slash and told him what I had and he is getting another buffer and spring out to me. I'm not knocking RR at all, but I did pay special shipping charges to get it installed, up and running. I have emailed the person I have been working with at RR, but didn't get a resonce back yet. I just want to add this to the llist of information on 308 AR's parts so that someone else doesn't do the same thing.  If someome is looking for a buffer and spring for a LAR 308 with a RR carbine stock, I would make them a good deal.

http://photos.imageevent.com/shadow40/myar/websize/IMG_2793.JPG

Things that will work on a LAR 308 so far from my experience-
*Daniel Defense 12" light rail with the right barrel nut for RR
*Mag-Pul grip
*Mag-Pul Trigger guard
*Mag-Pul UBR Stock with the correct spring and buffer
Smith Vortex 5/8" X24" TPI Flash supressor
And obviously a Night Force scope/mount supplied by Larue Tactical.

FYI-

-MP

I just had a similar experience experience with my LR-308......bolt carrier would go far enough to the rear to lock back, but would strip cartridges from the magazine.

My DPMS buffer is a hair under 2.5 inches long.  The issue ended up being caused by an incorrectly shipped buffer spring.
The proper 308 carbine buffer spring worked fine.

Well that is most interesting.

This means that RRA and Armalite both use an extended carbine receiver extension tube, and both tubes allow a standard length AR-15 carbine buffer (~3.25" ).     The DPMS carbine buffer and the slash CAR-10 buffer are both more like 2.5"  and correspond with an AR-15 length receiver extension tube–– which has the same internal dimensions as a UBR.

Unfortunately, there is just less good information on the RRA, as they are newer and less are out there.

One question left:    on that RRA buffer, can you throw it on a postal scale and see how much it weighs?   I may want to take it off your hands :-)


Thanks

I am getting my POF .308 lower built as we speak, so it is a blank slate. I will but on a UBR so which spring/buffer set do I need for that?

It is 4.8 ounces-I weighed it last night. Slash told me it was probably an AR-15 9mm buffer. I ordered a buffer and spring from him directly yesterday. He knows what he's doing.

-MP

By most accounts, you will need an Armalite action spring (which may need a couple coils cut off) and a stubby 2.5" buffer from DPMS or Slash.

The 3.25" buffers are no-go in a UBR.

I think in general we should just start calling buffers by size.   If we say a 2.5"  buffer or a 3.25" buffer, there is not much room for mess-up.   Springs are easier, as you can get the longer one and cut it.

Slash do you have these in stock? Also where is your website? I'm new here.

Thanks!

Thanks email sent