In order to reduce noise you have to pick a cartridge that is:
1. Slow, low sonic is good, subsonic is better.
2. All or most of the pressure is confined to the barrel, and very little escapes outside to create sound waves.
3. No muzzle device to reflect sound back.
4. Only shoot outside, in outdoor ranges or hunting grounds. Shooting in an indoor range magnifies felt sound.
5. Stay away from jacketed hollowpoint (JHP) or full metal jacket (FMJ) bullets, those are often light and fast. Get a lead round nose (LRN) bullet.
Final advice: I'd suggest using a .45 colt carbine with a heavy, slow lead projectile.