This is where the Bullpup design comes into it's own, it gives you compact weapon with range and hitting power.
Bushmaster M-17s (USA) 
Caliber: 5.56 mm (.223 Rem.)
Action: Gas operated, rotating bolt
Overall length: 760mm
Barrel length: 546mm
Weight (w/o mag): 3.72 kg
Maximum Effective Range: 600 meters
Magazine - Accepts all M16/AR15 magazines