REFERENCE: HK 416

The Heckler and Koch 416. The AR15 chassis has become the de facto system for most western armies. However it's has major flaws, the biggest being problems with it's direct gas impingement system which requires constant cleaning and lubrication to prevent jamming. H&K rebuilt the Colt M4 carbine from scratch improving everything from it's gas system to more accurate and robust sights. Basically an M4 on steroids. The M4 fires using a system that redirects gas from the expended round to eject it and reload another. The 416 uses a gas-operated piston that physically pushes the bolt back to eject the round and load another. Carbon buildup from the M4's gas system has plagued the rifle for years, resulting in some close calls with Soldiers in combat whose rifles jammed at critical moments.
H&K claims the 416 now has the rugged dependability that was once the sole domain of the AK47. The 416 can be submerged in water and buried in sand and still fire reliably and even upside down. Water on most other weapons would cause the barrel to explode from the temperature differential, but the 416 is able to dissipate it's hot gases away from the barrel and reciever.

The 416 is currently deployed with Delta operators in Iraq, but due to political reasons is not being adopted by the rest of the Armed services.