Making a nuclear bomb is rather easy all you need is to bring some uranium to critical mass by smashing together some sub-critical uranium. Shooting the two into each other and pressurizing a center by TNT explosions are two good methods (in fact they were used for the 1945 bombs dropped on Japan). So with the higher grade uranium you can cause a nuclear explosion by dropping two pieces together. Well, if you managed to get some very high grade nuclear material at just below a critical mass you could plate it with lead and use them as bullets. Smashing it into something hard could cause it to go supercritical. It would be plausible to make bullets which cause nuclear explosions.
You can't get far enough away to not be vaporized.
You can't get any uranium that high grade (at the very least you shouldn't be able to, and we should probably spend a lot of time making sure).
I'm probably wrong.
I've also come to realize that the force needed to launch the bullet initially should also perhaps make it go critical and detonate inside the gun. It would require a soft acceleration, much like one could accomplish via gravity and simple dropping out of an airplane. Although, railgun-like launches would also be feasible.