This article gives the Magma-Cannon usage tips.
To maximize its effectiveness, equip the Magma-Cannon with:
One Speed Mod (or none) is all that is needed to give the Magma-Cannon a great rate of fire. 4 Ammo Mods effectively double the ammo slots from 20 to 40. Impact Mods are the core of the Magma-Cannon, and are very necessary because it gives the enemy a shock that gives you extra time to fire off more rounds. Equipping more even might allow you to hit your enemy all the time while giving them no time to hit you back.
The Shock Mod works best on this weapon because it gives more damage to enemies in a wider range.
The Freeze Mod can give you time to fire off more rounds to groups of enemies or larger enemies, either slowing them down, or completely stopping them in their tracks.
The Acid Mods work on this weapon, effectively doubling or tripling its attack power. This allows you to conserve your ammo when trying to take down the larger enemies.