Here's the mechanics for fusils.
Trigger Charge. An arcane fusil requires no ammunition, but you cannot simply shoot it by pulling the trigger. The planarite takes a moment to gather the necessary energy. To charge the fusil, you spend a bonus action and pull back a firing hammer. At the start of your next turn, the fusil is charged, and can be used for a single attack.
The shot of an arcane fusil is either a pellet of flame that engulfs a target hit, or a shaft of crackling lightning.
Once fired, you cannot charge a fusil again on the same turn. It can only be fired every other turn.
If you charge a fusil but do not fire it on your next turn, the weapon suffers a misfire. Similar to a muzzle-loading weapon, you can clear the barrel by spending an action, but until you do the weapon has disadvantage on attacks. If you suffer a second misfire without clearing the barrel, the fusil explodes and deals its base damage die to you.
Burn. The fire fusil only deals a base of 1 fire damage, but the target also catches on fire. It takes 1d10 fire damage at the start of each of its turns, and can end this damage by using its action to extinguish the flames.
Shock. When you attack a creature wearing metal armor with a lightning fusil, you have advantage on the attack roll.