Because of the constraints of having to carry around stacks of physical ammo, most galactic armies and mercenary groups chose to use blasters, which instead fired plasma projectiles.
These plasma bolts were powered by highly compressed, superheated gas, making it possible to store hundreds of shots in a very small canister, and the type of gas used also determined the color of the blaster bolts. Tibanna gas was the most commonly used gas to power blasters, with the cheaper variants producing red bolts, the ionized version producing blue bolts which were especially effective against droids and even the highly refined version which produced invisible bolts.
Therefore, blasters could run out of ammo when their gas canisters were empty…