This is a Java simulation of an air-core inductor. Note that any effects of a projectile are not included.
All dimensions are in inches and the result is microhenries.
The simulator handles all necessary conversions to metric. The simulator will incidentally give you other handy information, such as the number of turns and the length of wire needed. Your goal in this simulator is to design a coil with a certain amount of inductance. You should have already chosen a target inductance value somehow, perhaps by experimenting with the RLC Simulator program. Here you can choose various wire sizes and physical dimensions to see what happens.