Monday, June 26, 2017

US Army R&D Wants to Radically Improve Machine Gun Barrels

The Firearm Blog, TFB, posted an article last week from an Army ARDEC (Armament Research, Development & Engineering Center) study on using additive manufacturing (3D printing) technologies as a new way to build rifled barrels; more than that, the big goal of the research is to see if the new manufacturing techniques can improve the performance and life of barrels.  The study was called "Optimization of Machine Gun Barrels Using Additive Manufacturing"  (pdf warning).  It's really worth a look if you're at all interested
The project’s goals as stated in the presentation were to eliminate the need for spare barrels to be carried by reducing barrel temperature (especially chamber temperature) and increasing the cook off limit of the barrel (the point at which a barrel gets so hot that rounds will fire from heat alone, without the primer being struck by the firing pin), without a decrease in accuracy or an increase in barrel weight. The team investigated two different 3D printing methods for manufacturing advanced barrel units: [Bold added: SiG]
The study, then, is to determine if additive manufacturing techniques allow them produce structures that are better than conventional barrels; primarily steels.

When heat is dumped into anything, there are only three ways to get rid of it: 
  • Conduction is by far the best way to get heat out of a system.  Examples are things that have been bolted to the hot object to carry heat away.  Forced water jacketed cooling is by conduction.  
  • Convection is generally a distant second.  Something like a computer CPU that has a heat sink on it, but no fan, is depending on the heat causing air currents across the heat sink to carry the heat away.  The heat sink just increases the amount of surface area that forces the air currents.  Forced air cooling, blowing a fan across that heat sink, is more like conduction cooling. 
  • Radiation - radiating the heat away by infrared radiation - is not usually a factor, although it can be when the temperature is very high.  When a barrel (or anything) is glowing red hot it's loosing lots of heat by radiation. 
Given that, it seems that what ARDEC is working toward is barrels that shed their heat so quickly that they don't build up heat enough for rounds to cookoff.  This is a "materials engineering" project; the essence is that the materials in each part of the barrel can be optimized for the required performance.  For example, a barrel might have material chosen for the barrel liner and first few hundredths of an inch away from the bore that wears very well and is very good at conducting heat into the next layer out.  The second layer may have been chosen for its strength but wouldn't stand to long term use if it was exposed to the wear of thousands of rounds going through the barrel.  ARDEC offers this road map:
  • Cobalt Superalloy Liner – we know these are good with respect to wear, also play some role in minimizing heat into the barrel
  • High Strength Core – minimize the thickness and weight of core
  • High Heat Capacity Jacket – maximize volume/mass
  • –Reduces overall temperature increase per unit of heat into the barrel
  • Optimize outer profile of barrel for increased convection
Machine gun crews have been carrying spare barrels since, well, the birth of the machine gun, right? I think it's a good idea for an R&D group to ask something industry asks every day, "we've always done it that way, but is it still the best way?" A lot of advancement in materials science and manufacturing techniques have come since the Maxim gun or JM Browning's first automatic.


  1. Stellite barrel liners were introduced during WW2 to extend the life of machine gun barrels. Today I think the need is more for heat sinks than anything else. It's the friction - heat that warps the barrels and wears them out faster.

  2. I don't have either the background, or the math skills, to figure out if embedding Thermoelectric Cooler materials in one of the barrel layers would remove significant amounts of heat via the Peltier Effect. Even if you used a resistive heater to waste the electricity generated, you would be moving it away from the bore and chamber.

    1. By definition, using the thermal differential to generate power would be less of a cooling effect than just conducting it (since TE devices are designed for low conductivity, otherwise, they would work less efficiently in a power generating mode). If you put power in, and used it as a cooler...then maybe, but only if the limiting factor was the heat rejection on the cold side and that were improved sufficient to make up for the worse heat flux through the TE vs through simply a high conductivity layer.

      Ideally what you would want is a layered structure with:
      1. Low wear, but high conductivity and high specific heat inner layer (to rapidly absorb transient heat)
      2. Then a high conductive but low specific heat layer to spread the heat out radially
      3. Then a very high specific heat layer to absorb the sustained heat
      4. Then a finned, high conductivity (and preferably high specific heat as well) layer to maximize the heat rejection.
      Ideally, the form of layer 2 would be a smooth inner surface, with a reticulated or finned outer surface, and the further layers would take this finned structure and increase the fin aspect ratio at each step, ideally with the fins being radial disks, rather than axial flute like fins to maximize the natural convection.
      Or, given the availability of very high performance extremely high speed miniature DC brushless motors, using axial fins with a forced air cooling...yeah, the fans are noisy, but so is gunfire.

    2. Oops, I should have read more carefully, as I just said exactly what ARDEC did, with slight variation.

    3. To put a different light on it than Marc did (excellent post, BTW, Marc), Peltier devices are diodes: semiconductors that allow current to flow in one direction. Peltier noticed the effect that one end got hotter than the other. The geniuses that design semiconductors figured out how to make the effect stronger, but the way a Peltier cooler works is you pump a lot of current through it and you blow a fan across the hot end to remove the heat leaving the colder end to get colder. Without removing the heat, the cold end doesn't do much.

      When I say "a lot of current", think of things like a single bottle wine cooler. Those use 12V at around 6 amps - 72Watts. A CPU cooler I looked at used 5V at 10 amps. To use a Peltier cooler would require a pretty big battery.

      While I don't know exact numbers, I'd guess you'd get more efficient cooling by just using that current to run a fan blowing across heat sink fins on the barrel.

    4. I use TECs to stabilize the temperature on image sensors all the time. The big thing to remember with a TEC is that is takes a lot of power to move heat, and then you have to get rid of the heat from the device you are cooling, the heat from the power you are dissipating in the TEC to move the heat, and the heat from the TEC controller. The amount of power required to move the heat is typically a lot more than the heat originally generated. Cooling a .5W image sensor by 30C, for instance, will typically require about 1.25W in the TEC, so now you have to get rid of 1.75W (ignoring controller efficiency). It actually makes the heat-shedding problem worse.