So you are running a carbine length gas system? It will not likely run suppressed? You will be fine with a non-adjustable gas block. Like you said, you can just put a heavier buffer and maybe use a spring that's better than mil spec. Something from Springco, JP, or I think Tubbs. I use Springco and JP. JP makes a nice flat wire spring that works well.
You can also go VLTOR A5 buffer system to help smooth it out. (but that's probably overkill for a 12.5" barrel) The A5 is a rifle length buffer system that works well on SBR's, and I have one on a 20" 556 and the recoil while running suppressed is almost nothing. When I get a tax stamp, it will get a shorty upper, so it will be interesting to see how that goes.
I have Colt 6933 11.5" SBR that I use suppressed. It has an A2 front sight block, and I put a JP spring and a Spikes Tactical T3 buffer (5.4oz) in it and runs great without an AGB. Recoil is not too bad.
I also have a LWRC 10.5" A5 SBR, and it has a 2 position adjustable gas block and an H2 buffer. Felt recoil on the LWRC is noticeably less than the colt, both running suppressed. The LWRC is a half pound heavier, so that will eat up some recoil, but the AGB helps a lot.
12.5" is a great length for an SBR in that you aren't compromising much at all to get a more compact system. You should be fine tuning it on the buffer end of the rifle. You might try to find the heaviest buffer that will cycle reliably and also lock the bolt carrier back, all while using the weakest ammo you might ever use.
Scott
