Firing Barry the Airship's Cannons

Barry the Airship features four cannons. They operate as follows:
Each cannon has three green gemstones on the back, indicating their charge. The cannon's capabilities vary based on how many charges it has, with increasing charge allowing for additional uses. Regardless of which option is chosen, all charges in the cannon are expended. If a cannon has more charge than it actually needs to use for the operation at hand, the gemstones turn red to warn that some charge will be wasted.

gems	effect
0	cannon must be charged before use
1	cannon can fire a small, lightweight object. Not useful for causing damage, but could conceivably be used, for example, to fire streamers, messages, etc. The projectile may way no more than 20 pounds, and is launched up to 100 feet away. This can optionally apply a feather fall effect, useful for transferring cargo to other vessels or dropping them down to land below the vessel
2	cannon can fire a heavy object that fits within the barrel at projectile velocities. This can be traditional ammunition, such as a cannonball, or something more creative, such as a dwarf. Living projectiles take 3d10 fire damage when fired. Damage from the attack varies by ammunition, but as a baseline a normal cannonball would deal 4d10 fire damage.
3	if empty when fired, the cannon will conjure a magical cannonball. Such a blast deals 5d10 force damage, or twice that vs. structures

There are three possible ways to recharge a cannon:

1. Consume any number of spell slots of any level. 1 gemstone is charged per two levels of spell slot. For example, you can apply 2 gems worth of charge by consuming one first level slot and one third level slot. Excess charge is wasted.
2. Expend life force by consuming two of your largest available hit dice per gem of charge.
3. Cannons can passively regenerate one charge for every eight hours of inactivity