Multi-cannon
A multi-cannon is mounted on one of your ship's hardpoints and needs assigning to a fire group before it can be used. These weapons are projectile based and use actual ammunition, which needs replenishing when depleted. They're basically a Gatling gun in concept with low power usage and heat signature. These weapons also have a greater range in general to their energy weapon counterparts. They fire from a magazine which automatically reloads when empty. Each multi-cannon that's been mounted, and is active, has its own pool of ammunition to supply it.
Buying Ammunition
When all your multi-cannon ammunition has been used up, you can replenish it by docking in a station and selecting the Fuel and Munitions menu. From the Munitions menu you can purchase a new stock of ammunition for your multi-cannons.