I haven't researched myself, but Speer and Hornady make 9mm 147gn pistol projectiles that expand at subsonic velocities, so I thought there may be a similar projectile in 308 out there (220gn?) for the Blackout.

Expansion isn't always required, as long as the energy is otherwise transferred to the game eg solids in dangerous game shooting. I've also used cast projectiles in my 458WM - 500gns doing about 1600fps (est) that didn't expand reliabally on pigs but stopped them outright. Same rifle with subsonics in 400gn killed rabbits instantly (without expanding of course). I wanted to try the subsonic 400gns on pigs, but never got around to it.