The game is mainly a horizontal scrolling shoot-em-up, although some scenes scroll vertically too. Shooting enemies will reveal power-ups and continually shooting them will either change their type or reveal a special weapon. Different weapons are more effective than others when fighting the end bosses.

Originating in the arcades, Side Arms is a horizontally scrolling shoot ‘em up in which you control Lieutenant Henry (a second player can control Sergeant Sanders in some versions), armed with crude weaponry and protective jump-suits. They must save the earth from the mercenary invaders of Bozon. You must first reach Bozon’s territories, then wipe out its forces hidden in caverns and lurking in underground seas, before destroying the main invasion ship. Lots of extra weapons can be picked up, often with similar tokens to those in 1943 and some other Capcom titles. Most weapons can be shot either to the left or the right, giving you more range to launch attacks.