The U.S. Army’s standard service weapon, the M4A1 carbine, has a defect that allows it to discharge a round without actually pulling the trigger. The flaw was caught on video earlier this year by a soldier at a shooting range, and testing by the Army has detected nearly a thousand weapons affected by the problem.

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In March, according to Stars and Stripes, a soldier recorded cellphone video of his M4A1 carbine discharging a round after the selector switch was moved. The selector switch on the M4A1 allows the user to engage the weapon safety (“SAFE”), preventing it from firing, firing semi-automatic (“SEMI”), or firing fully automatic (“AUTO”).

In the video, the soldier moves the selector switch between SEMI and AUTO. When the soldier further moves the switch to AUTO the hammer falls, sending the bolt carrier group flying forward and firing a bullet. Needless to say, the only time weapon should ever go off is when the trigger is pressed and this situation has the potential to be very dangerous.

Army Times says the Army has warned troops of the problem and is requiring them to perform a function check on their (unloaded) carbines to see if they can duplicate the error. If moving the selector switch without pulling the trigger causes the hammer to fall, the weapon is defective. At least 881 weapons or approximately ten percent of those checked have exhibited the problem and the Army extended the warning to older M4 carbines, M16A2, M16A3, and M16A4 rifles.

The Army recently started a conversion program designed to improve M4 carbines, upgrading them to the -A1 standard. The new M4A1 has ambidextrous controls, a heavier barrel, and a new trigger control group. The old trigger control group gave soldiers the choice of firing semi-automatic or three round burst. The new trigger group deletes the three round burst capability and adds full-auto fire.

It’s not clear that the new trigger group is responsible for the problem, however, especially since the Army has extended testing to older weapons. The problem may actually be much older but has just escaped detection. There is no practical purpose in placing the selector switch between settings.