Starting off, I have no interest in .308 and especially not in an AR. However, my brother decided to get into just that and he picked up most all the parts for one. My knowledge on .308 ARs is basically non existent because their lack of standardization turned me off to them. But, if it makes any difference the lower that he plans to put the UBR Gen 2 stock on is from Aero precision (he's sticking with their parts for the majority of the rifle).

Anyways, the waters are muddy trying to figure out which spring and which buffer we will need for this set up? And I'm also not sure if the included spacer that comes with the UBR Gen 2 would be needed or not?

I'm also uncertain on pistol grip compatibility with the Aero .308 lower, but it seems like Magpul has 2 different versions of the MIAD grip with one being specific to .308 lowers. So I'm assuming that they are not interchangeable?

Thanks for any guidance.

I used a kit from Slash Heavy Buffers for my UBR as well as an adj gas block. Mine works great but it’s on a Mega Maten set, I have no experience with Aero.

I got an Aero M5 buffer kit. I might have put a different flat wire 308 spring in, but I don't remember.

An adjustable gas block is probably more important though.

The UBR Gen2 comes with a longer A5/SR25 receiver extension. For an LR308/AR10, you can do a couple of different set ups.

1. Use a regular LR308 carbine spring and short 2.5" LR308 buffer. You must use the included spacer.

2. Use an AR10 spring and regular 3.25" AR15 buffer with whatever weight runs your set up better (H, H2, H3, etc...). Don't use the spacer.

Well if that's the consensus then that would clear things up quite a bit.

Thanks!

Hmm, now I'm wondering about this... Because with A5 length buffer tubes you need to use the longer A5 length buffers. You can't go and use the carbine length buffers in them or your carrier key will be impacting the lower receiver. At least that's how it would be with the AR15. Now with the .308 and different bolt carrier with different carrier key placement I'm not sure if that would be an issue or not?

The A5 buffer tube is nothing more than an AR10 length receiver extension. The AR10/A5 receiver extension needs to be longer for an LR308/AR10 to use the standard length (3.25") AR15 buffer because the BCG is longer than an AR15's.

For an AR15, the A5 system buffers had to be made longer (4") to work with the longer AR10/A5 receiver extension and shorter AR15 BCG.

Thanks for clearing that up! Makes sense, I was thinking that the different length of the .308 bolt carrier must have something to do with the ability to use standard AR15 carbine buffers in the longer AR10/A5 buffer tubes. Never having had my hands on any .308 AR parts leaves me to mostly guess.

So is there a good starting point as far as which AR15 carbine buffer to use for a .308 AR?

It'll depend on your brother's preference. If he's trying to go as light as possible, I'm sure he'll want to run a lower weight buffer and adjust the gas block to run with that buffer and whatever ammo he's using. He should try different weight buffers and see which one he prefers to shoot with, but the the most important thing is an adjustable gas block. It's a must. The majority of 308's are over-gassed and suck to shoot (even more so if they're having cycling issues). But once you get them dialed in, they're enjoyable.

Myself, I like to run H3 buffers. I like the extra weight behind the bolt carrier, plus it adds some extra weight to the rear to help balance the rifle a little more.  

Ah, now that I didn't know. I'm still not sure what route he's decided on as far as the upper, barrel assembly and gas block, so I'll have to let him know about that.

Thanks again.

I am running a similar setup.  Runs great and is rock solid.

Great, thanks for clearing that up for me.

@lwbyo1

Just been looking at my options for AR10 buffer springs and I believe this one from Aero would be correct for use with regular AR15 carbine buffers in the UBR Gen 2 without the spacer.

Just looking for confirmation, because I think I'm right, but I'm also just barely past clueless.

https://www.aeroprecisionusa.com/308-rifle-buffer-spring

That spring is for use with a 308 rifle buffer and rifle length buffer tube. What you need is an Armalite AR10 spring (part #EA1095). DSG Arms  and Primary Arms  both sell them, but they both look like they're out of stock right now. You could try contacting Armalite directly and see if they have one in stock to sell to you.

LMT also uses the same buffer system set up in their 308 rifles. Titan Defense has the spring  in stock. I just ordered one of the LMT 308 buffer tube kits  from them (1st time ordering from Titan Defense) for a build I'm going to be getting into soon. I'll get to see if there's any difference between the LMT and Armalite springs when it gets here.

Crap, thanks again! I'm glad I asked before I made any orders.

That LMT .308 buffer tube kit peaks my interest as I'm sure it's using an A5 length buffer tube and of course the correct spring to go along with it and the AR15 carbine buffer. Seems they went with the H3 buffer which makes me wonder if the H2 buffer I planned use would be adequate? Of course with an adjustable gas system it would probably make buffer selection less of an issue.

Not a problem. There are so many variables for the 308's, sometimes it gets confusing.

That's correct. An H2 buffer shouldn't be a problem since an adjustable gas block is going to be used.

Perfect, I sent links to my brother for the LMT spring and buffer (even though he might not need the buffer). Thanks again for the guidance!