I've been playing with a BO with a 9 12 inch barrel and YHM suppressor. I've found that hunting with this setup leads to a problem. To stay subsonic, I'm loading a 200 fmj Lapua bullet averaging right at 1,000 fps. At this speed, the bullet will punch a nice pencil sized hole through a hog without much damage........
I kill hogs regularly with a .223 fmj, and placed in the shoulder area they are one shot propositions, but they are traveling a lot faster when they hit the animal.
I love shooting the suppressed 300 Blackout, but I think I am going to relegate it to fun shooting and forget hunting with it.
You won't get any expansion with FMJ, regardless of velocity or bullet weight. And a 200 grain bullet at 1000fps will probably penetrate further than a 75 grain bullet at twice the speed. Upon contact, a light bullet gives up its energy much faster than a heavier one, less inertia due to lower mass.
Agreed with not having to shoot subs with a suppressor; obviously a suppressor will not decrease the sound of the supersonic shock wave, but it will substantially decrease the overall "footprint" of the discharge, and a 30-40 dB decrease is significant. My cousin and I have played with his 5.56 SBR (10.5" barrel) and my 300BLK carbine (16" barrel) shooting his gun with supers suppressed and unsuppressed, and subs suppressed and unsuppressed and my BLK with unsuppressed subs and with supers. The differences are pretty impressive, shooting his gun with supers and a suppressor, and shooting mine with subs unsuppressed. It's pretty close to the same, the difference is mainly in the pitch. His smaller caliber discharge has a higher pitched "crack", and mine is a lower pitched "pop", but both are noticeably quieter than unsuppressed supers from either gun.
The BLK's beauty is the versatility of the round, whether you're shooting from a short barrel or a carbine length. You can go from suppressed heavy subs from the short barrel, to lightweight supers from the carbine, and still have an effective platform out to a couple hundred yards for one and to around 400 yards for the other. Velocity does differ somewhat with the same load from the different barrel lengths, it's a matter of physics. The difference might not be as great as other calibers, because even the highest velocities are "only" in the low 2000 fps range with the BLK.
Most of the guys on the BLK forum who hunt seem to use something in the 125-150 grain range and drive them as fast as they'll go, to get the better expansion. Up to recently, most .308 spitzer bullets for hunting were designed for best expansion at velocities that the BLK just couldn't reach, but there are new ones coming out that expand better at lower velocities. I'm only punching paper and shooting silhouettes with mine, I typically load 155 Palma Match Kings to about 1850-1900 fps, and can knock down steel pigs at 300 yards, so they have a pretty good punch even at that range. I've only messed with subs enough to see at what powder load I can stay subsonic and still cycle my action. I'll eventually go the suppressor route, but now, I'm just having a ball with mine shooting middleweight supers.