yah i agree with everyone who has said anything about .223's, it's a pretty heavy charge so you get a lot of muzzle velocity out of it...but it's also good for plinking cause it isnt gigantic like a 30-06 or something like that. it also isnt super expensive, but that's only if you order bulk plinking rounds...sporting goods stores stock a lot of higher quality shit that you could use for hunting, but if youre just fucking around and sending metal downrange, you can pick it up in bulk online from military surplus suppliers and freaky overseas outfits. 7.62 is also hugely common and you can buy it pretty cheap in bulk, but it's a little pricier than the .223 ive found, about 20 cents/round
but yeah, you could get a .22, but those are only fun for slinking around in the woods with your friends when youre looking for squirrels and rabbits...if you want to punch a hole in a car axle at 50 yards or make a cinder block explode, get a larger bore rifle...it's just more fun