.50 cal BMG
the .50 BMG is really just an enlarged 30-06. BMG stands for Browning Machine Gun. It was used by a canadian corperal by the name of John Furlong in afganistan to obtain the longest confirmed sniper kill... he used a 750 grain Hornady A-MAX bullet and a Mcmillan TAC-2 rifle. the overal length of the shell and bullet together are just under 6 inches long.

And dont worry about a terrorist getting ahold of one of these and shooting down one of your airplanes, the M2 was a BMG machinegun and spat out 50 rounds a second. it was disbanded because it wasnt effective enough.
find good prices of .50 cal BMG rifles at www.edmarms.com/products/m96.htm

or
www.50bmgstore.com/

**hint: dont shoot this if youve never shot a 30-06 or .270 before