A 50 caliber bullet is actually .50 caliber, this means that the BULLET itself has a diameter of .50 inches. And a 9mm bullet is 9mm in diameter. The standard 9mm handgun fires a 9x19mm bullet the 19mm refers to the length of the CARTRIDGE as a whole (tip of bullet to base of cartridge), lots of guns are the same caliber or mm but have different overall cartridges. A good example is the standard NATO 7.62mm(.308 cal)x 51mm compared to the soviet rounds of 7.62x39(AK) and 7.62x54R (RPK,PKC,PKM, and Dragunov) R means rimmed, the cartridge has a lip or rim around the base. Or an Army M107 rifle uses a 12.7x99mm (.50 Browning Machine Gun BMG) round and a pistol uses a 12.7x33mm(.50 Action Express AE). Without the decimal point a 50 cal rifle would be shooting a bullet almost six foot in diameter(non-existent) plus tanks have very thick armor and use reactive armor, large steel boxes mounted to the exterior filled with 30 pounds of C-4 plastic explosives each. The science of this is that a large outward explosion will defeat a smaller incoming explosion or bullet. Please understand that I am NOT being smug or condescending or argumentative. As a former U.S.A. Sniper, I enjoy talking ballistics and putting out good information. Hope this helps.