The Lyman book of cast bullets shows a 400 grain bullet with 11 grains of Unique as going 1050 fps in a 45-70, which is under the speed of sound.
However, under the speed of sound, or sub-sonic, hasn't got a thing to do with it. No load that will get a heavy 45-70 bullet out the barrel will be "air gun quiet." The noise comes from the burning of powder and the gas escaping. Less powder, less noise, but any 45-70 load that will get to the 100 yard target will have a fair bang to it.
I found a light load that gave good accuracy with heavy lead bullets to be 22 grains of 4227. Lyman states this is about 1,200 fps. With this load the bang from the firing had completely faded before the smack of the bullet hitting the target came back.
I was once trying to photograph a bullet coming from a barrel. I set up in my basement with a 44 mag Marlin rifle, with a trigger device that would trip a strobe light. I had a block of wood to catch the bullets. When all was set I would turn out the lights, open the camera shutter, then shoot.
I don't have my notes in front of me, but I was down to something like 3 grains of Unique with a 250 grain bullet. Even that extremely light load made quite a crack. Enough that I put on my ear muffs in the basement. The bullet would about half burry in quite hard stove wood.
