Burning Out Machine Gun Barrels

When a machine gun is fired continuously for an extended period of time, evidently the barrel will "burn out." This I have learned from reading, not from experience.

What exactly happens when a machine gun barrel burns out? Are the lands of the barrel melted by the extreme heat? Or does some other type of damage occur to the barrel?

What would happen if an operator continued to fire an MG with a burned out barrel? Is there any risk of injury to the operator?

Approximately how much sustained fire (rounds fired, or minutes fired)would be required to burn out the barrel of the common air cooled machine guns of WW2? (Bren, Browning A4, MG34, MG 42, DP, Nambu)

 

Exactly what jeff said. You have to fire a sh*t ton of rounds through them to get to that point though.
"Burn out" was much more comon on pre-WW2 machine guns due to the type of powder used in the projectiles. The powder would leave much more residue in the barrel causing a myriad of function problems in the water cooled MGs of the time. When "Air cooled" variants became the standard these same problems manifested in barrel corosion and accelerated barrel wear resulting in loss of accuracy and unserviceability.
More often than anything an over heated barrel will cause the ammunition to ignite when it is chambered causeing a "run away gun".

 

Similar with mortars? Not being an infantry type...apart from shooting down the range and standing on guard..I only guess from books and movies...Plenty of tales of watering the barrels of both Machine guns and mortars.....Weeing on order in fact...

 

I've also read of the 5"38's being fired at a rate that there barrels were red and perhaps even drooping a bit.

 

http://www.youtube.com/watch?v=dGAwrmOapb4

http://www.youtube.com/watch?v=Kzfm4pYhIyY

 

That's just silly

 

Heat treating various steels is a controlled development to achieve desired result. Shooting a machine gun until it glows actually results in recrystallization of the steel's structure which is uncontrolled and in all probability produces weakened strength. A preamble to failure of many types.. Why the MG 42 had a quick change barrel. Experienced machine gunners short in "burst" and just did not hold the trigger back. Attempts to dissipate heat in air cooled MG's can be seen in perforated shields and rings on the barrel to give more radiating area but air cooled machine gun will fail quicker that one would think without shooting burst . Early M-60's , in trials , began to fail after 500 rounds on continuous fire, two 250 round belts together.

I have never heard the term "burn out" applied to machine gun barrels but that means little, I have never heard of many things !! LOL, I have heard it applied to corrosive wear of a barrel throat just ahead of the chamber in often shot high velocity guns such as the old 220 Swift. Applied to machine guns it could mean "cook offs", expansion of the chamber which could split the case allowing gases to escape through the breech leaving a projectile in the barrel, various distortions of parts, failures to extract, etc. lots of possibilities.

The MG 42 was a fearsome weapon but could not be fired at 1100-1200 rpm for long. Short burst and barrel changes eat into it's specifications. The slower firing MG 34's were often fed from a 60 round drum, still a gunner in a JU 88 would have been busy changing them out but also cooling his barrel.
And good old john Browning shot 20,000 rounds of 30'06 through his water cooled MG to demonstrate it;s reliability.

 

ANY gun can overheat, simply because of the hot gases and especially the friction of the projectile as it flies down the barrel. Machine guns are the most prone to it because they fire so fast, but even a semi-auto is at risk if you fire enough rounds fast enough. Some people will get the barrel of a Mosin-Nagant hot enough to be painful to touch.

Another problem of barrel overheating is that the hot barrel exerts more friction on the bullet as it's fired, which means more energy is wasted (causing a loss of power and accuracy) and even MORE friction to heat up the barrel. It's basically a self-feeding problem that can only be stopped by not shooting any more. The heat also affects the moving parts, which makes it more likely that you'll suffer a malfunction or even break a part as the metal loses its temper. The more you fire an overheated gun, the faster it wears out.

Plus the obvious problem with closed bolt weapons, where an especially hot chamber may cause a "cook-off" and the gunpowder spontaneously combusts. If you're lucky, it's just a random discharge. If you're unlucky, you were firing full auto and the round detonates before the breech is fully sealed and you lose some fingers to an exploding gun.

 

I'd like to see some documentation on this. Something about it doesn't quite sound right.

 

The Barrel expands when hot; it does not contract and exert more friction on the round. The expansion of the barrel leads to greater windage and escape of the hot gas propellant, which leads to lower muzzle velocity and, since the bullet is rattling around more as it goes down the bore, less accuracy.

TM9-2190, the tech manual for the M3 50 cal, states that the barrel will overheat when subjected to 200 rounds of continuous fire. 6-9 round bursts are mandated.

 

Thanks that sounds more reasonable.

 

Never heard of drooping but have seen the them glow at night---))))

The 5"38 had a rifled liner. After so many rounds the rifling would be worn smooth and accuracy would
greatly degrade. During the latter stages of the pacific comapaign the 5"38's were severely tested and
raarely failed. Damn good weapon.

 

I am surprised to learn this. I would have thought that the barrel expanding would mean it getting thicker, and therefore reducing the bore diameter?

 

Aargh. There was a better version of this from years back. Deja view all over again.

 

Plenty of bent barrels from SF GPMGs in the Falklands

 

Actually I believe the barrel gets a bit thinner as both the internal diameter and the external diameter will increase. You might be able to test it by taking a washer and heating it up with a propane torch. I'm pretty sure a micromenter could measure the difference but that might ruin the istrument. Dropping it briefly on a piece of wood my leave a scorch mark that could be measured or simply compared against the washer when cooled.

 

Think of it this way - the interior circumference of, for example, a .30 caliber barrel is about .9425". That circumference can't get any smaller as the metal expands, can it? The interior diameter and circumference, the thickness of the barrel, and the exterior circumference would all increase.

 

I've read that the the number of rounds that each 16" barrel fired was recorded and the number was entered into the mechanical computer used for fire control so that it could compensate for barrel wear.

And I think they wore out pretty quickly. If the same bore diameter/barrel lenght ratio was used in a 30:06 rifle, it would be like getting 2500 feet/sec out of a 15" long barrel. So an extremely hot load. I don't think any hand held rifle can come close to that performance with that bore/barrel lenght ratio, and if it could, it wouldn't last very long.

Edit: That was for a 16"/50

 

Geeze, that is hot...How many rounds before replacement?