BrickArms M4 Carbine
Currently in use by US troops, the M4 is a shorter and lighter version of the M16A2 assault rifle, achieving 80% parts commonality with the M16A2. The M4 has selective fire options including semi-automatic and three-round burst (like the M16A2).

Fires 5.56 x 45 mm NATO cartidges at a rate of 700–950 round/min fed from a 30-round detachable box

$1
 
(Disclaimer: All BrickArms tiny toy weapons are made of solid ABS plastic, designed to be used with LEGO toys. They cannot shoot bullets and cannot be made to fire bullets in any way. Again, they are a TOY! T-O-Y Toyee!)