Debating on whether or not to get a can for a 5.56/.223 and 300 bo. I know absolutely nothing about what ammunition I can and/or cannot use shooting suppressed. I use Wolf Gold .223 now for the 5.56/.223. I don't hunt and I'm looking for plinking ammunition.

I would go with a 30 caliber suppressor, then you can shoot both 5.56mm and 300BLK through it. Now 5.56mm is always going to be a little loud due to the supersonic crack of the round. While with 300BLK you can buy subsonic ammo which won't give you the sonic crack, making it quieter and more pleasant to shoot. Of course you're giving up ballistic advantages by going with subsonic ammo. Supersonic ammo is of course going to do more damage to a target if using it for hunting for home defense.

I've shot about 50 different factory 5.56/.223 loads through a suppressor without issue. Everything from cheap 55gr wolf steel to Black Hills 77gr TMK and it's all been fine.

For BLK, I've shot at least 15 factory supersonic and 10 factory subsonic with nothing but good results.

Just get a .30 suppressor and whatever ammo you want to shoot and you'll be good to go.

Just make sure your bbl will stabilize whatever you're shooting.  Baffle strikes are no fun.

He could also shoot subs in 223, so, not “always louder”.

Look into the SilencerCo Saker 762 ASR or the Omega. I run a Saker762 on a 300BLK SBR, a 16" 556, and a 24" .308. Great suppressor, recently switched out the mount to the ASR and I am pleased with it so far. I see no point in running subs with 556 as you might as well be shooting 22LR. As for 300BLK look into some of the heavier loads offered commercially (208-240gr) or roll your own if you reload.

I currently shoot 208gr AMAXs in my subs out of a 9.5" barrel. Sounds really great with my can.

What 300 bo ammo is GTG? I'm looking at Magtech 123 grain although for suppressed shooting I'm not sure.