A friend wants to build an AR based on a 18" FN Upper, in 5.56 NATO. It's his first build, and I'm advising him. He'd like an adjustable stock on the gun.

The spec on the upper says it has a rifle-length gas-system, but doesn't give a measurement. It may be 12", but in the product image it look like 10".


What length receiver extension (buffer tube) should he use (Rifle or Carbine)?

If you advise a carbine length tube, what buffer weight?

Thanks for your time, and attention,
Storm

I wouldn't install a carbine action on anything.
While a carbine action will work I would use an A5 if you want a telescoping stock.
The A5 action uses an intermediate length tube, rifle spring and an A5H2 buffer which is the same weight as rifle buffer.
I used a Vltor A5 kit on my 18" and pieced together the others.
The A5 buffer tube is a .308 buffer tube.
Spikes has them for $20 or so the last I looked.

BCM also has complete kits.
https://bravocompanyusa.com/ar...l-management-system/

Either of those options are a good choice. I run A5's on my 18-20 rifles. I also run one on a 16"

quote:

Originally posted by powermad:
I wouldn't install a carbine action on anything.
While a carbine action will work I would use an A5 if you want a telescoping stock.
The A5 action uses an intermediate length tube, rifle spring and an A5H2 buffer which is the same weight as rifle buffer.
I used a Vltor A5 kit on my 18" and pieced together the others.
The A5 buffer tube is a .308 buffer tube.
Spikes has them for $20 or so the last I looked.

Thanks for your advice, powermad.

I saw those A5 tubes online, and read up a little on them. I was previously unfamiliar with them. Will an A5 tube will accommodate any mil-spec adjustable stock (like say a Magpul CTR for a 5.56)?

Also, thanks for the BCM reference.

As long as it's mil spec then the ctr will work.
I dont think they even have commercial size tubes as an option.

He can get a Magpul UBR gen2, which has a built-in buffer tube which is the correct length for the A5 system. All he would need is the stock, an A5 buffer (A5 H2 buffer is the logical first choice, but the system is tunable; there are lighter and heavier A5 buffers available) and a rifle-length action spring.

Thusly:



There are less expensive options and there are lighter options, but if you want a collapsible stock that has the rigidity of a fixed stock, and has a consistent cheek weld no matter how much or little you have the stock extended, then this is the way to go.

An excellent explanation of the A5 system: https://sigforum.com/eve/forum...450002025#1450002025

.

If you are starting from scratch on parts, I'd go A5, I can feel the smoother cycling over a carbine system.

You might have to hunt a bit to find the buffer you want right now (the price of tungsten weights has gone up 500% this year).

I ran a standard carbine length buffer tube and CAR-15 stock on my 18" Mk12 clone with no issues. I used either an H2 or H3 buffer, can't remember as I've since cut it down to a 14.5" RECCE clone.

Who needs all that extra barrel anyway...

quote:

Originally posted by parabellum:
He can get a Magpul UBR gen2, which has a built-in buffer tube which is the correct length for the A5 system. All he would need is the stock, an A5 buffer (A5 H2 buffer is the logical first choice, but the system is tunable; there are lighter and heavier A5 buffers available) and a rifle-length action spring.

Thusly:

[Image Removed]

There are less expensive options and there are lighter options, but if you want a collapsible stock that has the rigidity of a fixed stock, and has a consistent cheek weld no matter how much or little you have the stock extended, then this is the way to go.

An excellent explanation of the A5 system: https://sigforum.com/eve/forum...450002025#1450002025

.

Thanks for the recommendation and the video, Para.

I have a UBR on a gun. I like it. It was originally what I had in mind for my friend's gun. Knowing now that UBR's essentially have A5 tubes, I think this may be the way I'll advise him to go.

The original UBR will not accept an A5 buffer. It is only the second generation UBR.

quote:

Originally posted by parabellum:
The original UBR will not accept an A5 buffer. It is only the second generation UBR.

Thanks for the info on that. The UBR on my gun is close to 20 years old. It wasn't my intent to "A5" that gun. Good to know, though.

I found an Vltor A5 kit (w/o tube) that I hope will work. I've advised my friend to go with the UBR and that. The buffer weight (5.3 oz) sounds like an it's A5H2.

Vltor A5 kit:
https://www.midwayusa.com/prod...003408914?pid=148116

As long as the tube is intermediate length that is the kit.

When I wanted to go with A5 I already had a leftover 308 tube and a couple of rifle springs, just needed an A5H2 buffer but at that time there were none available anywhere for a long time.
I found the only stock kit with an a5h2 buffer on gunbroker and it was the last one in Forest Green.
So I wound up with the whole thing for just the buffer.
When the buffers finally started to show back up I picked up a couple and assembled a couple more.
Swapped out all the A2 tubes for telescoping stocks except for my 308.

quote:

Originally posted by Storm:
Vltor A5 kit:
https://www.midwayusa.com/prod...003408914?pid=148116

It's ridiculous the prices they're asking for this stuff these days. I guess that's the only way to get one of these buffers these days. All he needs is the buffer and the spring.

Glad I don't need a lot of AR parts these days. The prices are obscene.

Just for reference, I went back and looked at prices I paid at Midway for Vltor buffers.

In December, 2016, I bought an A5 H3 buffer for 39.99.

In April, 2018, I bought another A5 H3 buffer for 44.96, an A5 H2 buffer for 40.46, and an A5 H4 buffer for 53.96.

Those last three, I bought as spares, and they remain in their sealed packaging still.

I have two other A5 H2 buffers and I think one or both of those came in the form of that kit currently on Midway's site, but I can't find the receipts. I think when I bought them, they were half the price of what's being asked now, but I guess, considering price increases in everything over the last decade, that's not surprising.

For those who may be interested in the UBR Gen2 Midway has the black or FDE models on sale for $35 to $40 (w/free shipping) less than most other vendors.

quote:

Originally posted by parabellum:

quote:

Originally posted by Storm:
Vltor A5 kit:
https://www.midwayusa.com/prod...003408914?pid=148116

It's ridiculous the prices they're asking for this stuff these days. I guess that's the only way to get one of these buffers these days. All he needs is the buffer and the spring.

Yes, I noticed that. I bought a standard H-buffer last year, and thought the price was a bit high. Now, the prices are indeed ridiculous!

After going through several iterations on what to recommend to my friend, going with this recoil assembly for the UBR stock was the least expensive and easiest option. I just hope when we get this gun put together, it runs well and he doesn't have to be tuned up.

It'll be fine.