.45 Colt round ball.
.454" 144 grain muzzle loader ball lubed with liquid Alox seated deep to the case mouth - over 3 grains of Trail Boss.
Out of my 20" carbine they are very quiet, about like a .22 rifle, and 1" accurate at 25 yards. Still respectable at 50 yards, 3"-4", but accuracy and trajectory drop off the cliff after that.
thinking about a wad and filler system to fill the void between the base of the ball and the powder. I might be able to get down to just 2 grains or so of Clays or similar and reduce the noise even further.