Quoted:
I found what I think is a steal on this can and while I shouldn't spend the money would I be a fool to pass it up? I plan on using it on 300BLK and limited .308 (20" barrel). While I realize it isn't as quiet as the RC no supersonic ammo through a can is that quiet to me.
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My quick take:
We know that the 556 RC is somewhat of a compromise on sound already. I have one, run it on SBRs, and am happy to trade a few dbs for weight, mount, POI, etc.
I've seen people that run the 7.62 mini on on 5.56 SBRs suggest they are a bit disappointed in the sound results. I imagine you'd find the same on BLK - where sound reduction is a huge part of the equation - and probably on your .308.
Silencer Shop has the full sized 7.62 RC right now for $1,210. These are still probably louder than peers.
SF is currently testing a .300 BLK designed can. Don' t know if it will work on .308 as well.
I'd go full size 7.62 RC, or 7.62 Saker if I were you, but you have to decide what is best for you.