the coil is wrapped around the aluminium pipe will it effect the efficiency of the coil gun ?
I think it will make it totally non-functional. Since the aluminum pipe will act like a shorted turn and aluminum happens to be a very good conductor, the pipe will absorb most of the flux produced by the coils and turn it into electric current and heat. With the result being almost no flux gets to the projectile. So if you consider going from 90% to 0.05% a significant effect on efficiency, then yes. The pipe must be made out of a non-conductive material. Or at the very least, it needs to be cut so there is no complete path for current around it's radius.
Yes, it absorbs most magnetic force generate by coil and turns it into useless current .
