When I did it I used the old (weren't then of course) pellets like these...
Yeah normally I use just use 'standard' lead pellets, but on special occasions and for testing I used to break out the Paragon. At around £5 for 100, they're not cheap.
Are they all hi-tech now?
No, you still get lead pellets as standard. These newer generation 'air bullets' are just a bit of a specialty. They aren't new though, I remember seeing some old air rifle magazines from the '70s/'80s and they were available, then, in some form or other.
A .357 air rifle? Wow that's some pellet lol
There is actually a UK company that makes air rifles in anything up to .50 caliber. Of course it's pretty ridiculous though, as the amount of energy you need just to give the 'pellet' the same relative ballistics as a 12 ft. lb. firing a .22 is massive.