I would go .300 blackout in a really short barrel. I have one with a 6” barrel and Maxim PDW stock. It’s a lot of fun to shoot. My .30 can for it is still in jail, so I haven’t been able to shoot it suppressed yet.

I had a .308 SBR (HK51) years back and it was a completely different beast. Huge fire balls and muzzle blast. I sold it and would never go that route again.