It is worn on the arm, and has a wire that connects it to the scouter (some soldiers opt to wear helmets in place of scouters, in which case this wire reaches into a port in the helmet). By pressing a button on the side, it fires some sort of energy. It is unknown if the energy fired is the same as ki energy.
During a flashback of Majin Buu attacking Planet Alpha, the Alpha-seijin try in vain to kill Buu with Arm Cannons that fire pinkish purple beams. Only in the anime version of Dragon Ball: Episode of Bardock, Toobi, Cabira and Chilled's other soldiers use Arm Cannons.
Video game appearances
It is used by Frieza's soldiers in Dragon Ball Z: Goku Gekitōden and Dragon Ball Z: Sagas. In Dragon Ball Z: Battle of Z, Beam Gun is an equipable item that increases Ki Blast attack power for three minutes.
In Dragon Ball: Xenoverse, it is a Ki blast Super Skill called Beam Rifle used by Galactic Frieza Army foot soldiers Raspberry, Navel, Monre and Gupure. Due to the skill's requirement of an Arm Cannon, it is one of the few Super Skill that the Future Warrior cannot obtain or learn. Raspberry (and his palette swaps Navel, Monre, and Gupure) can also use an Arm Cannon-based variation of the Double Galick Cannon as their grab/throw.
It appears under the name Beam Strike in Dragon Ball Z: Dokkan Battle.