
@ Graham Crocker
Yeah, but to get the slug to move from a dead stop to mach7 (2.3 Km/s as is pointed out above) in half a second you'd need a barrel 575m long.... (if I can remember my old mechanics lessons....)
I should imagine you'd need to get that down to more like 10 or 15 m as a maximum, and that's going to be a very very short pulse.... Not so easy
Why Paris? Why not :-D