Suppressors, Snails and Sub-Sonic Ammo.....
Suppressors are legal with BATF application approval and $200 tax stamp in most states. Not CA, however, so you're going to have to "suppress" your whole shop/range. Materials are available for exactly this purpose. Also, steel baffles and snails will stop bullets better and more safely on a range. Ventilate to avoid sucking in a bunch of vaporized lead along with powder smoke.
The sub-sonic ammunition intended for use with suppressors and other applications greatly reduces the "crack" that accompanies faster bullets. For .22 rimfire you can get powderless and CB rounds that are quieter than some air rifles. For centerfire handguns keep bullet velocity below 1050 feet per second and you'll reduce sonic crack.
You might also check out some of the truly bad-ass air guns available these days. I have one that recharges off a scuba tank and delivers cartridge arm performance. Good luck.
