I'm running a 5.5" BA barrel, with a 14.9 oz faxon bolt.  The buffer I'm using a a Vltor A5SH1 (if I recall right), which weighs 7.19 oz.  I like the Vltor because they are right at 4" long and the weights are springs loaded so they are always where they are supposed to be.

I've found that the spring matters little in terms of recoil.  Where it makes the most difference for me is in the recovery, meaning where the red dot is relative to the target after the bolt SLAMS home.  I'm using a Blue Sprinco spring.  My dot comes right back down on target.  Too much spring and it will have your muzzle dipping.

Too much buffer and you're gonna slow things down too much.  I don't know how fast you shoot, but too much buffer can lead to your out running the trigger.

I also limit the travel of the bolt as much as possible.  I space by buffer so that there's about the thickness of 2 quarters between the bolt face at its point of farthest rearward travel and the bolt catch.

It's too bad that that this whole AR9 thing still hasn't reached any sort of standardization.  I tend the try to use only as much reciprocating mass as needed to insure a good lock time and keep whatever mass you have from moving more than it needs to.

Good luck.

And, sometimes the ramp works.

I want to thank everyone here that posted, it made a huge difference for me. Especially the links that were shared.

Topic Moved

From amphibians suggestions (check out his site https://c3junkie.com)
My blowback 9mm has carbine tube, kynshot RB5015 buffer, and tubbs 556 flatwire spring. Very smooth cycling.

My 9mm cmmg RDB also A5 tube, tubbs 556 flatware spring, but uses kynshot RB5007 buffer. Also very smooth.

@Droppoint  you still think the KAK 8.4oz Buffer is a good option?
I'm building up a Foxtrot Mike lower (I know; not your fav) with the matching 7" upper.
Think this is a good starting point for me?

Thanks for all the info already shared!

@ar15-joe

Yes, I think the KAK buffer is a good option, paired with a standard 5.56 recoil spring.  

If you're as OCD as I am, you can check the buffer "rattle". The weights should move about 1/8".  I noticed my 10 oz. KAK buffer may not have the necessary movement inside for the deadblow weights to work properly.  This problem seems to affect a lot of my buffers, both 9mm and 5.56, and I'm starting to think it's a chronic problem in the industry.  I put an article up on my site recently about it. Blowback9.com. I'm hoping to get a slow-mo camera soon to do some real testing.

The FM9 lower and upper is fine, I'd just stay away from their buffer/spring recommendations.  If the barrel gives you issues with feeding, talk to Rudy at Macon Armory for his feed cone treatment.  It's worth every penny IMHO.

Good luck with the build!

Thanks, man.
I actually read (and enjoyed) your articles about the buffer weight spacings and also your FM9 recommended changes.
Great work and thanks for sharing, too!