Also helps if the projectile is subsonic.
My favorits silenced weapon is the grease gun. On full auto, it sounds like a sewing machine, the action is the loudest part.
A 300 and 338 Whisper® sub-machine gun has been developed which renders obsolete any "pistol" caliber sub-gun. Accuracy and control are simply unmatched by any other subgun. A pistol has also been developed. Both can be suppressed. The 6.5 Whisper® (85 through 160 grain bullets) and the 7MM Whisper® (100 through 185-grain bullets) are ready for sale. Both work in AR-15/M-16 as well as other type guns. The 338 Whisper® (175-300 grain bullets) is extremely accurate and may be had in Contenders or "308" bolt action rifles can be converted to it. A switch barrel 338 Whisper® .308/7/08 can also be had. The 458 Whisper® uses all normal 458 bullets plus 560 and 600-grain low-drag bullets for tremendous downrange performance. The 500 Whisper® is simply the most powerful subsonic round in existence. Both of these can be had in bolt actions or Ruger #1 rifles. Numerous other very accurate sub-sonic rounds utilizing heavy ballistically efficient bullets have been developed. Those with special needs are invited to inquire.
Some may dispute the development scenario; other claim several of these cartridges were around in limited availability since the mid '60s.
Any rifle can fire subsonic ammunition, provided that you can buy it. Catalogs have plenty of such .22 ammo, for example.
I would think that one would want to stick with the heavier grain bullets that would have more punch at lower velocities.
A quiet 45/70 might be fun for night hunting of coyotes?