I've read that the 50 caliber rifles are legal to own, but difficult to shoot because of their one-mile range.
I'm told that the 50 caliber Barrett rifles are painful to shoot (you can taste blood in your mouth from the shock wave!), and the bullets cost $7 each, too pricey to family use:
To make the trip for distances over one mile, special powders are neded to get to 2,700 fps and you need to fill the shell with no airspace, the bullet resting on the powder.
Sandia Labs has announced a guided missle in 50 caliber!
Note that small space for gunpowder that limits muzzle velocity to 2,400 fps.
The four inch bullet has missle fins and is self guided, zeroing-in, based on a fixed laser beam, which is managed by the spotter.
In the two seconds for the bullet to travel a mile, the terrorist may move, but as long as the spotter keeps the beam on the target, this new missle-bullet promises to be accurate to six inches at a half mile.
Hence, this weapon is a two-man operation, but it is remarkable that Sandia has developed a technology that allows a bullet that is rotating at 200,000 RPM to steer itself!