Ok,

So I'm building 2 SBRs in 300 AAC (Blackout).  This is my first foray into this caliber and I have a few questions.

I am using 2 Aero Precision lowers:  1 is and M4E1 lower and the other is the Gen 2 Ambi lower.

I am using 2 Aero Precision uppers:  1 has a 8 inch barrel while the other has a 10 inch barrel.

My question revolves around buffers and buffer springs:  Should I use an H2 buffer for these rifles?

Can I use a regular carbine buffer spring or is there something else I should be considering?  Would a braided spring work better?

Any advice/suggestions would be helpful.

Thanks.

Going to be shooting supers or subs and will you be suppressing?

Supers and not suppressed then standard carbine spring and H buffer should suffice. Subs and/or suppressed might go with Spikes STT2 or H2.

Probably going to mostly be supersonic and un suppressed.

I'm still trying to understand the whole dynamic....

Why would I want a heavier buffer for slower rounds (sub-sonic)  that would seemingly produced less recoil?

I use an H2 in all my blackouts, sub/super/suppressed/unsuppressed any combo. The reason you’d need a heavier buffer is when it’s suppressed there’s more gas coming back so it pushes the bolt back harder.

I ordered a Colt H2 and a set of TTI carbine buffer springs.

I was wondering if there is any benefit to the Strike Industries flat buffer spring?

Interested to see how this shakes out.  I'm working on a .300 BLK as well, my second build.  I'm using an Aero M4E1 enhanced upper, with 8" barrel and M4E1 lower.  Intent is to get a suppressor and shoot mainly subsonic so after much research I went with an H2 buffer.  Still trying to decide on gas block.  Will you be using an adjustable?

Here's a trick... Buy a pair of CAR buffers (cause they're dirt cheap) and a handful of individual tungsten weights. Then you can mix and match the number of tungsten weights in each buffer. It's literally just a roll pin that holds the bumper and weights in. Easy peasy. Don't bother buying pricey H2/H3/XH buffers. I get the Expo Arms buffers cheaply and they work just fine.

Tune the guns by varying the buffer to just lock back on an empty using full weight 220gr factory subsonic loads. Then it should cycle supersonic and suppressed loads just fine, too. The .300BO cartridge and specs and design was made so that rest of the setup should use standard spec parts; regular carbine spring, a standard weight M16 BCG, and unregulated gas block/FSB. So assuming your barrel port diameter is proper, you shouldn't need to play with adjustable gas blocks or anything like that. I've done an 8" pistol, 10" dissy/kino style, and 7.5" pistol ... all worked wonderfully with factory loads, factory spec parts, and plain gas blocks/FSB.

I have found through a lot of problems over the years is that if you are running a non-supressed 300Blk upper that it is darn near impossible to over gas! The biggest problem is that most barrel makers assume that you are going to run it suppressed and make the gas port accordingly smaller. If the gas port is less the .090 in diameter get the gas port opened up to at least that if not larger in a pistol length gas system. That is why most of the builds have cycling problems. The other issue used to be trying to get a shorter barrel to use a carbine length gas system... it wont work!

For those shooting subsonics suppressed, seriously consider going to a JP Captive Spring system or the like.  This kind of recoil system entirely removes the "sproinnnggggg" of the carbine spring and buffer.

I built my own custom captive spring system with delrin clad buffer so there is no metal-metal contact inside the buffer tube.  It makes the rifle a ton of fun because it is so much quieter for the shooter.

For my captive spring system I built different removable buffer weights to tune the recoil system to the rifle and ammo.