A requirement for these vehicles was to have firing ports for troops riding inside, and so it was decided that a specific weapon also be developed.

Initially the FPWs had no locking mechanism and had flip up sights, along with a metal wire stock akin to that on the M3 submachine gun, to allow the weapon to be used outside of the vehicle more effectively.

Later this was dropped, and a new hand-guard introduced with a screw type locking mechanism to fix the weapon into the port.

Or, alternatively, Army officials omitted the feature to discourage troops from employing the M231 in lieu of the issue M16 rifles.

These weapons are only capable of fully automatic fire, with a special side plate mounted inside the lower receiver and a selector that only has "safe" and "auto" positions.

These included a note about the excessive muzzle rise of the weapon during firing, that the barrel collar would become hot when firing and should not be touched, that hearing protection should be worn, and lastly that a brass catching bag or evacuation hose system would have to be installed.