Your 9mm and 40 cal pistols probably won’t cycle with BP. Those are high pressure rounds, and BP is low pressure. Your revolvers will work just fine, but yeah, velocities will suffer some. Out of a rifle, your velocities will boost back up but you’ll need either semi-jacketed bullets or hard cast with a gas check to keep the gas-cutting and leading to a minimum. Or else run a wad behind the bullet.
All-brass shotgun shells are still made, and last almost forever so long as they’re cared for. You seal them with something called “waterglass”. Do a search to find out how to make it. Waxed cardboard wads can be fashioned from milk cartons or cartons of Half and Half. One thing I need to invest in is a shot pan…. and a buckshot mold.
The wicked flee when none pursueth..." - Proverbs 28:1