AR15

         The AR-15 is a lightweight assault rifle with a rotating locking bolt, actuated by direct gas impingement gas operation or long/short stroke piston operation.

        The AR-15 has many versions including semi-auto and selective fire variants, most of its components are eighter aluminium alloy and other synthetic materials, which explains its light weight.

        The AR-15 was originally designed by ArmaLite as a small rifle for US armed forces how ever due to financial problems the design was sold to Colt.
       The main working mechanism of the rifle is known as direct gas impingement, the gas is taped directly form the barrel as the bullet moves past the gas port which is located directly under the base of rifle's front sight, the gas then moves through a tube located above the barrel, the gas moves all the way back to the rifle's front receiver. Here this gas is used to push the bolt carrier hence reloading the rifle. 
       This operating mechanism is fundamentally different from the piston operated rifles. 

      
       It uses cartridges .223 remington, 5.56 Nato, 300 whisper, 7.62x35 mm and .22 LR. 
 Giving rate of fire of 800 rounds/min, muzzle velocity of 975 m/s, give effective firing range of 400-600 m. 
It also has various versions of magazines of 20, 30 and 50 rounds.