The main reason lies in the gas system. The “rifle-length” gas system of a 20" barrel is 5" longer than the “carbine-length” gas system used on all 14.5" and many 16" M4-style carbines.

The LTE weighs in at 5 lbs., 5.1 ozs. with an empty magazine. The 16" long solid steel, round-profile barrel is the same one used in this company's PRO series.

The gas block functions as an important part of the M4 and M16’s direct impingement system; it funnels gas into the tube and back into the bolt carrier group in order to cycle the weapon.

The M4/M4A1 5.56mm Carbine is a lightweight, gas operated, air cooled, magazine fed, selective rate, shoulder fired weapon with a collapsible stock. It is now the standard issue firearm for most ...

Both Innis and West said trading in the M16's 20-inch barrel for the M4's 14.5-inch barrel does sacrifice some muzzle velocity, which translates into a slightly shorter effective range ...

Translation: the M4 barrel is so unreliable that special operations forces units need to swap it out with a barrel from a different gun -- and one that actually works in real combat conditions.

The improved rail will also have to include a low-profile gas block that could spell the end of the M16/M4 design’s traditional gas block and triangular fixed front sight,” the report says.

The improved rail will also have to include a low-profile gas block that could spell the end of the M16/M4 design’s traditional gas block and triangular fixed front sight,” the report says.