The Magpul UBR should be a carbine length buffer system, but since it's a lot heavier than a CTR or other lightweight stocks, the UBR will eat up a bit more recoil. Plus it's very ergo and comfy to shoot with IMO. The Magpul PRS is a rifle length system and will soak up even more recoil with the extra inch or so of buffer travel. I also have a VLTOR A5 buffer system which works well at reducing recoil, and is worth looking into. The A5 makes my 556 feel like a soft 22 when suppressed. It's laughable how soft shooting it is. A suppressor will be a nice help in what you are trying to achieve.
I would think the PRS or the A5 would be my top 2 choices.
You are building everything in to the rifle to make it as soft shooting as possible, so the next area would be the ammo you would be using , a lighter weight pill could make it even softer yet.
What receiver set are you using?
Scott