Performance Firearms and Thermodynamics. Part 2

by Adam Devine

In Part I of this article, I talked a little bit about materials and their thermal conductivity. The gist of that first piece was to illustrate the importance of material choices in high performance machines—among which we would like to count firearms—where good thermal management is critical.

Does it even matter to the average shooter? For target shooters and competitors, I’d say it matters. For someone like a military sniper, who may need to place a long string of shots with extreme precision, I’d say it matters a lot. For hunters, who only need one or two predictable shots, maybe it doesn’t matter much. But maybe the peace of mind is still worth something.

It has been my experience that many mass-produced hunting rifles--bolt actions included--will give ambiguous results during zeroing. Assuming four shots with a scope adjustment, it's not unusual to have a three shot group of 2 inches, give or take a little. If that's adequate to your task, then you can pack up. If you must know with more certainty where that first cold bore shot will go, you'll have to wait half an hour or more for the barrel to cool completely. A light-weight barrel that walked less and cooled much faster would be worth something to me. My time is valuable.

In Part I, I mentioned that traditional coolants like air and water actually have pretty lousy thermal conductivity compared to most metals. We need to revisit air, because it plays a decisive role in our “static” firearm cooling systems. First, a bit of science speak, and then we’ll be off to the races again.

Thermal conductivity is measured in more than one set of units, but in the SI system it is expressed as watts per meter-kelvin. For our purposes, understanding the expression is unimportant, other than to note that it affords us a useful means of comparison between materials, so I will drop its use immediately. But to give you a sense of scale, here are two materials at opposite ends: diamond, at the very highest end of the scale, has a thermal conductivity of 1000, while at the lower end of the scale blown glass insulation has a mere .04. So diamonds transfer heat 25,000 times faster than insulation. Bet your wife didn't know that.

(CHART: Thermal Conductivity of Common Materials)

While we're comparing, here are a couple of numbers for the exotic materials I mentioned in Part I: titanium has a 22 rate. Carbon fiber at 150 tops can vary greatly depending the resin used and the arrangement of fibers and fillers, but the low end (traditional resins and fiber planes) is about 7 and the high end is around 150. Not bad on the high end, but still nowhere near as good as much cheaper aluminum alloys which can get into the low 200s, and whose thermal performance increases with temperature.

So where does air, our precious barrel coolant, fall in the scale? Get ready for disappointment. Depending on atmospheric conditions, static air is about .02-.024. Worse than most materials sold as insulation! In fact, most insulating materials are little more than a means of creating dead air space, because dead air is a superb insulator.

To be fair, air is hardly ever static in our world. Like most fluids, it moves in currents, thanks in large part to temperature gradients. But it is disturbing nonetheless to realize that the one coolant that firearms can rely on is actually one of the world’s worst. As we talk about thermodynamics in gun barrels it’s important to remember that air is effectively a barrier, unless we can move large quantities of it.

This is one of the reasons that rifles competing in accuracy have very thick barrels. Compared to air, steel at a rate of 80 is a very good thermal conductor. Since heat moves across gradients, more steel creates a larger gradient, effectively “soaking up” heat at the hot bore and moving it outward toward the surface of the barrel, where it meets—ugh—air, coming to a screeching halt. Once the gradient ceases to exist, ie the barrel’s temp is normalized, it is up to the tediously slow process of air cooling to bring down the temp of the barrel.

Fire five shots through your rifle in quick succession, and you will feel the exterior of the barrel get warm to the touch in just seconds, thanks to the fast acting thermal transfer of steel. Now wait for the barrel to return to “cold.” It can take 30 or 40 minutes. Because air sucks. If this was not the case, your barrel would cool as rapidly as it heated. Look at it this way: in terms of thermal gradients, your barrel steel is a late summer brush fire. By comparison air is a thin, patchy lawn.

In light of this we must consider a means of speeding the transfer of barrel heat to ambient air. Luckily, we do get a little help. A warm barrel creates its own thermal currents in the surrounding air. As we know, warm air rises, so the heated air right next to the barrel’s surface will rise, setting up a current that curls lazily around the barrel, drawing fresh, cool air from beneath. The larger the gradient, the stronger this current will get, resulting in more rapid cooling from high barrel temps. But at smaller gradients this current is paltry.

Barrel fluting has become pretty popular these days. Does it help? Let's look at an example. I have a fluted DPMS AR-10 barrel (take-off) that will serve our purposes. This barrel is 1.05" under the hand guard, with six .25" flutes cut for 7" longitudinally. We'll skip the middle school math and go straight to the results. These six flutes, cut .125" deep, expose 6 square inches of additional surface area on the barrel. That sounds pretty good. But the barrel's total surface area without flutes is 52 square inches. The increase due to fluting is about 11.5%. Not very impressive. Air cooled cylinder heads have fins that double or triple their surface area, and without forced airflow they will still overheat.

Another thing to contemplate: The material removed during fluting is steel, having a conductivity rate of roughly 80. In its place is now air, with a conductivity rate of .024. Hmm. That extra steel could have helped normalize your barrel at a slightly lower temp than the barrel whose mass has now been reduced by fluting. Also, flutes are nearly always cut longitudinally, but hot barrels do not create longitudinal air flow. They create vertically rising currents, and longitudinal flutes can effectively become dead pools of air, like eddies in a stream. Oops. You’ve traded a good thermal conductor for pretty little pockets of insulation.

It’s true that flutes reduce barrel weight. In our example barrel the reduction is about 4.5oz. It’s also true that flutes do not reduce rigidity quite as much as turning an equal amount of mass from the barrel’s whole diameter. But alas, it is also true that flutes introduce machining stresses into the barrel that otherwise would not have been there. These stresses can raise their ugly head at surprisingly modest temperatures, often limiting, rather than increasing the number of shots in an accurate string as the barrel “walks” earlier. It’s for all these reasons that some premium barrel makers will not flute their barrels. If you must have a fluted barrel, be sure that it has been stress relieved after fluting. This will generally not be the case with mass produced barrels.

So what can we do with all that heat generated in the bore, without simply adding more and more steel to the barrel? The options are few, and without a dynamic cooling system a barrel will always reach a temperature saturation point limited by its mass. I for one am not ready to rule out dynamic cooling for small arms.

Liquid cooling is a non-starter. While water transfers heat 26 times faster than air, it is still far worse than most metals. Without circulation and a heat exchanger it will simply be a barrier to heat transfer. There are materials far superior to liquid that can pull heat away from the hot bore. Aluminum, for instance, conducts heat more than twice as fast as steel. Copper has a phenomenal 400 conductivity rate.

Copper's other properties make it a poor choice for barrel use, but aluminum, in combination with traditional barrel steel, could make a lot of sense. This avenue has its own detours, like differing expansion rates, harmonics, and extra machining, but a well engineered, cost effective solution could have real merit. Aluminum is quite rigid, exceptionally light, affordable, and can pull heat away from the bore very rapidly, potentially increasing barrel life and accuracy at once.

Once the heat has been whisked away to the barrel's surface, we still need a means of dissipating it. Air sucks, but it's all we've got. Let's move it. Semi automatics use combustion energy to operate mechanical systems. Why can't one be a forced air system? Even a little forced air would be a big improvement over the modest thermals created by the hot barrel. And, if we force the air along fins that seriously increase surface area, then we've got an actual cooling system.

There are challenges inherent in such solutions, and nothing will be as cheap as a basic chro-moly barrel. But I believe good engineering and modern manufacturing methods can incorporate some real improvements without exotic pricing. Doubtless, early adopters can expect to pay a premium price, but with consumer awareness and sales come economies of scale. There could be a high performance barrel in your future.