ARmy Rumour SErvice

**HE117** · 13-05-2012, 14:56

Your biggest problem with achieving light weight with a machine gun is heat management...

The higher the energy of the round, and the rate and length of fire, the more heat you have to get rid of. If you allow the chamber to get too hot, you get serious breech erosion and increase the chance of a cook off. Modern materials will help to some extent, but not that much. You also greatly increase the risks of cook offs and breech explosions, which may be ok in an aircraft weapons bay or a gun turret, but not with a manned weapon..

If you abandon metallic cartridges in order to lose weight, you are just making the problem worse.. A major secondary effect of using metallic cases, partiularly brass, is the removal of heat energy from the system. Non metallic cases simpley do not have the thermal capacity of metal. Brass is a particularly good heat conducter, steel less so.. the best would be silver, so the Lone Ranger had it right all the time! Plastics are poor and caseless is well.. caseless!

Unless you come up with some better way of tranferring heat away from a gun barrel other than air conduction and radiation, there will always be a limit. In the 1900s, when metals were more heat sensitive, they used liquids to cool the barrel (Vickers-maxim) or ducted air (Lewis). These got abandoned in the 1930s with the development of high speed steel and high temperature alloys such as stellite, but these limits were reached fairly quickly..

I don't think you can realisitcally increse the maximum operating temperature of a machine gun much above current practice. The danger of cook off is still there and frankly with so much TI on the battlefield, a white hot barrel is really not the way to go..

Perhaps we need to bring back external cooling again...?

Anyone for an 8.6x70 Vickers? - At least you would get hot water for the tea!

**ugly** · 13-05-2012, 15:01

Get the brews in then!

**incendiarycutlery** · 13-05-2012, 16:50

The way to get improved cooling would be to utilise high thermal conductivity combined with phase change.
The Vickers used water -> steam phase change and steel is pretty thermally conductive, plus water convects so you can heat it pretty evenly.

I think that it might be possible to achieve a more modern solution. Aluminium conducts far better than steel and modern phase change materials have a higher latent heat and tailorable phase change temperatures.

**lastfreeman** · 13-05-2012, 17:14

Hmm, there was that lone inventor Ward who invented Starlite heat resistant plastic. He got on "Tomorrows World" with it, but guarded it & died with the secret. Then there are those heat resistant alloys used in jet engines. Though I am trying to avoid exotic materials that would make the gun too expensive.

**incendiarycutlery** · 13-05-2012, 17:51

How expensive is too expensive?
What is exotic?
These things are all relative. Bear in mind that there is most likely going to be a multiple-thousand pound electro-optic sight put on top of the gun and you will be feeding 200 round belts through it at 50p a shot or so. At the same time £60,000-per-shot Javelins are being used.

**stoatman** · 13-05-2012, 18:09

You don't want to be heat resistant - you want to absorb and shed as much heat as possible. Al has a good specific heat capacity, but is not very dense.

Forget anything other than steel for barrels. There are various vacuum treatments you can do to coat and/or modify the surface, but further little you can do. A Ti barrel is currently basically impossible to make economically.

**lastfreeman** · 13-05-2012, 18:48

The Soviet Union toughened aluminium by adding Scandium.

**stoatman** · 13-05-2012, 19:06

Originally Posted by lastfreeman

The Soviet Union toughened aluminium by adding Scandium.

So does Smith & Wesson. What's your point? Can't make a barrel out of it. (and it increases the yield stress. Dunno about toughness, which is not the same thing.)

**incendiarycutlery** · 13-05-2012, 19:21

You could sleeve a thin steel barrel in aluminium to take advantage of the higher thermal conductivity, lower weight and higher bending stiffness of aluminium vs. steel, although it would be fun to deal with the different coefficients of thermal expansion.

**Buzz** · 13-05-2012, 19:27

You couldnt do that.

The two materials have a different thermal coeffecient so you would end up with a warped barrel as the two materials try to expand and contract at different rates.

Aluminium in fact has only one third of the stiffness and density of a similar volume of steel so it would make a pretty shit barrel for sustained fire use even if the bore is a steel liner.