For backyard practice one can always go to the extremes of the Cat Sneeze loads. One uses a very light well lubed lead bullet and tiny amounts of powder, with a bit of TP holding it against the primer.
The most extreme Cat Sneeze I tried that uses powder is about 2 grains of Unique under a lubed round ball.
The most extreme quiet experiment I have done so far is a 115 gr lead M1 carbine cast bullet
In a case drilled out to accept shotgun primers. It was nearly silent, quieter than my air rifle, I can swear I could see the flight of the bullet, and it made for a satisfying side yard paper punching round.
I did not test for penetration as I only wanted cheap quiet paper punching trigger time on the rifle.
Quiet Roundball gallery loads allow me trigger time on the actual firearms, really magnify my mistakes in technique!
I intended to try similar experiments in .45 Colt and .45-70 but have not had the time yet.
Prof Marvel
Be careful, check that the bullet left the barrel, beware of barrel rings, etc etc