A few questions about overheating machine guns

Questions from watching this video:

  1. The machine gun stops firing because it runs out of ammo. But by that point, you can see that the red-glowing heat is creeping closer and closer (it’s glowing the barrel, and the heat is getting closer to the shooter.) Suppose it didn’t run out of ammo and just kept firing indefinitely, would the eventual interior barrel melt as well?

  2. Is it still possible for an overheating machine gun to fire rounds by itself due to the barrel being hot enough to ignite the cartridges, even without pulling the trigger?

  3. The suppressor melted off midway. Suppose a round had struck the melting suppressor, would it ricochet and hit the shooter?

  4. Why does the suppressor glow hot long before the barrel itself does?

  1. Yes, it’s called ‘cook(ing) off’
  2. Because the suppressor is made of much thinner metal than the barrel.

No way to know without doing the detailed calcs.

The barrel and in fact the whole gun is radiating heat to the outside world. The hotter it gets, the more effectively it radiates. Meanwhile, each shot is adding heat. The whole assembly gets hotter as long as each shot adds more heat than radiates during that time. As things get hotter, that excess of input over output gets smaller and smaller, eventually reaching zero. At which point the whole system is in equilibrium.

How hot is that equilibrium temperature and will the barrel have melted first? Probably. But we won’t know without doing a lot of math.

More realistically other problems will likely occur first, like a mechanical jam from parts expanding unequally or the cook-off that @crowmanyclouds explained.

Given the frequency of bullet passages, it’s a good bet the softening suppressor was hit more than once. But that would deflect the bullet towards someplace, not necessarily directly back at the place where the shooter is.

The heat is going to start visibly warping the barrel before it starts melting. Many machineguns are designed with replaceable barrels precisely because over-heating of the barrel is an expected consequence of sustained rapid fire.

One anecdote from my personal experience. My Reserve unit was doing its annual qualification on the range. Everyone had finished up, and we had quite a bit of live ammo left. Apparently turning ammo back in required significantly more paperwork than simply certifying that all the ammo had been used up, so my unit leadership encouraged anyone who wanted to do so to just shoot off as many rounds as they wanted.

I took a couple of magazines, and just for the heck of it, set my selector lever to 3-Round Burst (the M16A2 doesn’t have full auto), and fired as quickly as I could pull the trigger. I forget how many magazines I fired off that way, but I think at least two or three. When I exhausted my ammo, I set my rifle down on the rest by the firing position. The rifle rest was a thick PVC pipe sticking up out of the ground. You put the rifle barrel on the edge of the pipe ring, with the muzzle pointing down range. My rifle barrel melted a notch into the PVC pipe. I was a little concerned by that, but the unit armorer certified that the barrel was still true, so apparently it didn’t get hot enough to warp.

My dad was a bomber pilot in World War II. He told a story of a training exercise in which the waist gunners were firing too long. The barrel of one gun melted, and a stray bullet blew a 2-foot hole in one of the airplane’s tailfins.

Dad used to joke that he flew 50 combat missions without a scratch, and then the worst hit he ever took was self-inflicted.

They would cook off but that’s why machine guns are designed as “open bolt”. When you let off the trigger the bolt locks open with the chamber open to cool off. It’s also why some machine guns were water jacketed to cool better.

I’d imagine the bullet would just tear through the thin, soft metal of the melted suppressor quite easily. But it’s not something I’d recommend trying.

Full auto AR-15 meltdown