300 BLK subsonic is way too much fun.
For hunting, don’t just use any projectile - it needs to expand at these low velocities. Maker and Lehigh defense make excellent expanding subsonic projectiles. The 190 SubX is more affordable, but the performance seems lacking from reports. Sierra now has a 205 gr expanding projectile as well.
I’ll be using the 200 gr Maker this year on whitetail does.
For plinking, I usually use blems from Midway or Shooters pro shop. Midsouth even has some 220s available as well.
For brass - either form your own from 5.56 (just watch vids on YT) or buy some with factory headstamp like Starline and others.
When loading for subs and testing charges at the range, work your way down from higher charge to lowest. Otherwise, I’d your charge is too low, you risk having a squib in your barrel.