Rheinmetall has been awarded a patent (#8225999)for an invention (concept would be a more accurate term) to limit a machine gun’s rate of fire over a period of time.

This M4A1′s gas tube melted after 911 rounds of continuous fire.

All gas operated machine guns fire at a rate which is unsustainable over longer periods of time. The operators need training to ensure that they do not let the gun overheat by firing to many rounds in to short a period of time. This issue received national media attention when it was revealed that M4 Carbine barrels had melted and SAW machine guns had jammed during the Battle of Wanat.

From the Army’s report on the battle …

Specialist Bogar fired approximately six hundred rounds at a cyclic rate of fire from his SAW when that weapon became overheated, and eventually jammed the bolt forward. Specialist Stafford noted, “Bogar was still in our hole firing quite a bit. Then Bogar’s SAW jammed. Basically it just got way overheated, because he opened the feed tray cover and I remember him trying to get it open and it just looked like the bolt had welded itself inside the chamber. His barrel was just white hot.”

According to The Ground Precautionary Message ACALA #97-031 (emphasis added)…

(3) BURST BARRELS RESULT WHEN THE WEAPONS ARE FIRED UNDER VERY EXTREME FIRING SCHEDULES AND THE BARREL TEMPERATURE EXCEEDS 1360 DEGREES FAHRENHEIT. WHEN THE BARREL REACHES THESE EXTREME TEMPERATURES, THE BARREL STEEL WEAKENS TO THE POINT THAT THE HIGH PRESSURE GASES BURST THROUGH THE SIDE OF THE BARREL APPROXIMATELY 4 INCHES IN FRONT OF THE CHAMBER. THIS CONDITION CAN RESULT IN SERIOUS INJURY.

…

(D) SUSTAINED RATE OF FIRE FOR THE M16 SERIES RIFLES AND M4 SERIES CARBINES IS 12-15 ROUNDS PER MINUTE. THIS IS THE ACTUAL RATE OF FIRE THAT A WEAPON CAN CONTINUE TO BE FIRED FOR AN Indefinite LENGTH OF TIME WITHOUT SERIOUS OVERHEATING.

The actual rate of fire of an M4 Carbine is 700 – 950 rounds per minute, over 60 times higher than what the gun can tolerate over an extended period of time.

Aside from catastrophic failures, some guns are inaccurate when overheated. The Bundeswehr recently published a report saying that the H&K G36 was useless beyond 200 meters when it was overheated.

Rheinmetall proposes adding a power supply, microcontroller and actuator to machine guns to slow down the rate of fire to keep the sustained rate of fire inside acceptable parameters for the weapon it is installed on. Each time the operator pull the trigger it will fire, but the rate it will cycle is constantly adjusted.

I think this idea has merit but there are some fundamental problems. The foremost problem is that the circumstances where a machine gun might be fired continuously is rare and so operators are unlikely to appreciate a heavy electronic device messing with their rate of fire the rest of the time. They will soon figure out how to disable it and at most it will become a heavy paperweight.