Depends. A 50 cal can be a fully automatic machinegun used by the US military, often mounted on tanks or light vehicles. A 50 cal could be a long range sniper rifle, like the Navy SEALS use. Could be a long range benchrest rifle used in competitive shooting, typically at a range of 1000 yards. Could be a Desert Eagle semi-automatic pistol, often seen in the action movies. Or it could be a single action revolver, used for big game hunting and target shooting. The term 50 caliber simply discribes the diameter of the bore of the barrel (1/2 inch).