.22's are the best.
I have a semi-auto .22 to practice using semi-automatics (Browning Buckmark), a .22 revolver to practice using revolvers (.22 heritage), and a .22 Jennings to practice using a "mouse gun"
There's nothing like spending $20 in ammo, going shooting all day, and still come home with ammo, very dirty guns, and a big
on your face. The best time that one can have for under $21 dollars IMNSHO