Pre-charged air canisters for high pressure/caliber pellet guns will be useless after the shtf. Unless you got a way to make that air compressor still work, and even then a firearm would be better than going to all that trouble.
They have high speed pellets these days which are light, and look a little bit like a sabot slug (even though I believe they retain the jacket). I saw them over at dick’s sporting goods, suggests that they have 30% more velocity.
Also you can shoot a BB out of a pellet rifle. Its funny how a pellet gun can almost seem like a muzzleloader. You take a piece of paper or some other thing to hold the bb in place much like a wad, but not compressed enough to become a barrel obstruction. Maybe just a small ripped up piece of paper that goes flat in front of the bb and the bb pushes into the barrel a slight bit (maybe about 6mm in width, the size of a pinky finger nail). I did this alot as a kid. The steel BBs weigh less than pellets and will go faster.
In the case of bb’s you can use steel or copper clad ones, but for the intended use of a pellet gun id get a BB-caliber Shotgun shell, rip it open and get a handful of Lead BBs from it.