Longbow has a bore of 9 inches, remember. This means a shell with diameter of 7-8 inches, and the shell has to be about half a meter long. This would also mean that the mass is at least 10x as much as a standard shell, and hence WAAAYYY more momentum. The shells also come with GPS guidance systems and quick-action retractable fins, so it can correct its path and stay airborne longer. Also, we made the gun proportionally longer and therefore there's more electromagnetic acceleration. If I were to make a guess on how far the range would be for a kinetic shell, it would be around 2300 miles in which case it has to be fired through a thinner part of the atmosphere and glide there for a while.