Weismann’s Gravity Cannon

You touch a non-magical crossbow or similar object and temporarily enhance it with the power of dunamancy. The object becomes a Gravity Cannon capable of launching metal ammunition at incredible velocity. For the duration you may use your spell casting ability modifier for the attack and damage rolls, and you ignore the loading property. Firing the cannon consumes 0.5 lbs of iron/steel and deals 5d6+modifier force damage 

the spell ends prematurely and the object reverts to its non magical state if the cannon leaves your hands.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the damage increases by 2d6 for every two slot levels above 4th.