So, whether it’s the Minimi (M-249) or FN MAG (M-240) or most other machineguns made since the grand daddy of them all (MG-42), machineguns often use a spare barrel to reduce overheating.
This comes at a cost. Swapping out the barrels creates a temporary period of vulnerability, rate of fire is reduced, operators can get burned handling the barrels, the barrel can get lost (especially if you leave a hot barrel on snow!) and the gun has to have looser tolerances to allow barrels to be changed easily, which reduces accuracy.
Instead of having two barrels which can be swapped out, why not have a fixed barrel which is twice as thick?
I have a few hypotheses but I’d like to hear what Dopers have to say.
A thicker barrel will have virtually the same surface area as a thinner barrel. So the amount of heat lost per second would be virtually the same. The fact you’ve got to heat more metal up will increase the time before you have to stop shooting, but over the long term won’t significantly increase your rate of fire.
But with swapping barrels, you have twice the surface area to cool from. Meaning a higher rate of fire is possible without over heating.
I was a submariner so I have no idea why infantrymen do the things they do, but I would imagine it would be easier for four riflemen to carry three pound barrels in addition to all the crap they normally carry, rather than two riflemen carrying six pound barrels.
I’ve heard that the entire squad is built around the machine gun.
It has been a long, long time since I was a soldier and even longer since I have messed around with the two principal machine guns of my time, the .50 cal M-2 and the NATO round firing M-60. Of those two only the .50 cal routinely had its barrel changed. Since the .50 cal was usually mounted on a vehicle of some sort carrying a spare barrel was not a big deal. Generally there was also a heavily padded glove that went along with the weapon so that the gunner could extract the hot barrel without frying his hand.
There were three things to remember about the .50 cal:
Cock it with the palm up and the thumb out,
Never remove the back plate when the weapon was cocked,
Always head space the weapon before firing – the head space is the gap between the face of the bolt and the back end of the barrel. A gage for the purpose was supposed to be attached to the weapon by a light chain but it was often lost. In a pinch the head space could be checked using a Nickel and a Dime. It the Dime would slip into the space and a Nickel would not, the space was correct, the bolt would fully chamber the cartridge and the first round fired would probably not blow up in your face.
Surface area only goes up with the square root of the mass for a gun barrel. So twice the surface area, four times the mass. Probably not a great bargain. There is a point where the surface area is enough to keep pace with the energy input, but until you reach that size - whatever it is - you just put off the time when it overheats, and don’t eliminate the problem. One would assume that experience is that two barrel works.
A really good high emissivity coating would probably be the best first cut optimisation.
Back of envelope: wouldn’t a barrel twice as thick use much more metal (>4x)? The military likes to keep costs down (ha?), and would go for the cheaper option here.
One reason not mentioned yet: an overheated barrel can warp, rendering it useless. Obviously, you’d want to change barrels before it gets to this point, but that doesn’t always happen. If a barrel does warp, you’ll want replacements on hand. So it would seem to me that multiple barrels are a necessity for a machine-gun crew regardless of the weight.
Many guns use an outer piece of metal drilled with holes for two purposes:
it stays cool enough to reduce the odds of burning yourself on it
it does speed heat radiation by increasing surface area.
Here’s a sample of Google Images that show what I’m talking about.
Not trying to hijack the thread, but would like to ask you a question since you have fired the .50 caliber. From what I have observed the .50 is fired by pushing a button on top of the handle. How does the trigger mechanism work? Meaning, in a rifle you pull a trigger that mechanically sets into motion the action that releases the firing pin. In the 50 you are pushing a button, how does that translate into the gun firing? Does the button connect to a rod or something in the handle? I am assuming it isn’t electronic. Just curious.
There are also models of the Browning M2 that have a trigger. In both cases, the pressing of the firing plate or squeezing of the trigger releases the sear, which allows the hammer to contact the firing pin, launching the first round down-range. The rest is standard auto fire until the plate, button or trigger is released.
If the goal is to speed up heat loss, wouldn’t a thinner barrel be more desirable than a thicker one? Thinner = less metal = less mass = less heat capacity = faster cooling. Consider a sheet of aluminum foil vs a cast iron pan.
As I thoght and others said, two elements are desired: slow heat build-up and fast cooling. Slow heat build-up is a function of mass (ceteris paribus) and heat loss is a function of area.
You want to maximise heat capacity and heat loss in a barrel.
Dracoi, I thought the barrel shroud was mainly to allow grasping the front of the weapon without buring your hand. Sure, holes in the shroud allow the barrel to cooler faster than if you had a shroud without holes, but I’m not sure a shroud with holes cools faster than no shroud at all.