^I think that owning your own high intensity infrared laser, aka "death ray" is surprisingly legal in the United States. And yes they do exist.
I am not sure how rail-guns qualify under current firearm laws. They can be scary powerful though. 2800 fps is a low powered rail gun. 2000-3500 meters per second is a much better velocity for a good rail gun.
I think that you could build a rail gun that would move your 7.62mm projectile at 2800 fps if you really wanted to. Although you are going to be working with capacitors and voltages higher than you should really be messing with unless you know what you are doing. Its the sort of thing that can electrocute you to death in an instant if you do not know what you are doing.