Performance Firearms and Thermodynamics. Part I

by Adam Devine, Ranger Point Precision

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Unless you work as an engineer, you probably haven’t given much thought to thermodynamics since your last physics class. If you bring up the topic whilst hot tubbing, people will start putting their clothes back on. It is a fascinating subject, but perhaps a little dry for hot tubbing. In industries focused on performance machines, thermodynamics is a vital science. If you’re in it to win it, ignorance of the subject will result in your complete annihilation.

Having spent fifteen years in the high performance motorcycle industry, I am now puzzled by the reality that the firearms industry has neglected this science for so long. After all, two truths are, or should be, understood by now: 1) firearms generate a lot of heat; and 2) heat is a barrel’s worst enemy. So why has so little attention been given to disposing of that heat? To make matters worse, the industry itself often misinforms consumers and muddies the waters. But I’ll get back to that. For now, let me illuminate some basic principles, drawing on my background in high performance motorcycles. Guns are essentially single stroke engines, so there are plenty of analogies.

Compared to the firearms industry, motorcycles evolve at a breathtaking pace. It wasn’t always that way, but these days there is an enormous amount of money at stake. Honda, far and away the global leader, sells roughly 15 million units annually. They and their competitors understand that success on the race track sells bikes. So these are very serious battles, not just by the riders, but by the hundreds of engineers and support staff behind the riders. The most serious arena of all is Moto GP, the premier class, equivalent to F1 in auto racing. Factories will spend hundreds of millions to compete, and find some technological edge, at this level. And not just motorcycle factories either.

Tire companies like Bridgestone and Pirelli will also spend millions in development to ensure that their tires are on the winning bikes. Tire engineers study thermodynamics a lot. They are the unsung heroes of the sport. The best engine, chassis, and rider will get nowhere near the podium without a reliable way to transmit all that power to the tarmac. And all that power transmission to the tarmac generates—you guessed it—lots of heat. Tires have an optimal operating temperature. Too cold and the rubber compound doesn’t deform well enough to grip the road surface. Too hot and the compound can become “greasy” and unpredictable, or deteriorate too rapidly to keep the rider in the race. Tire warming jackets are used in the pits to ensure that the optimal temperature is achieved before the bike hits the track. Thermodynamics taken very seriously.

Tires aren’t the only parts to which the science applies. Engines are an obvious focal point. They too have optimal temperature ranges, and high compression race engines generate an overabundance of heat. Brakes get hot enough to turn water instantly to steam, and they too have to dissipate enough heat to remain at an optimal performance level. Exotic materials are everywhere.

Exhaust systems are an outlier. While they have an optimal temperature, that range is essentially as hot as possible. It is desirable to retain heat in the exhaust system. Hot exhaust gas maintains its velocity, helping to evacuate the cylinders for the next charge. Cooling exhaust slows the process. Here again, exotic materials can help.

Titanium is one of those prized materials. It’s used in racing exhausts for the same reasons it has been used so extensively in supersonic aircraft. Most people know that it is light and (some grades) strong, but few realize that it is also prized for its low thermal conductivity. The latter makes it a very poor choice for any gun part that is in contact with the barrel. Titanium barrel nuts, for instance, are a new fad for ARs, but they’re a bad idea. They insulate the barrel right where it’s generating the most heat. Oh, and despite the goofy insistence of pop culture, titanium is not bullet proof.

Another exotic material made popular in racing is carbon fiber. It too is prized for its light weight, strength—it’s very rigid—and thermal properties. In exhaust systems, it is often used in the construction of mufflers. It’s a superb insulator, helping maintain exhaust core heat, with the added benefit that riders and passengers don’t scald their legs on exposed mufflers. This should be a tip-off that, despite its rigidity, it’s a poor choice for barrels.

There are companies making carbon fiber wrapped barrels. By most accounts the good ones shoot quite well. In .22LR applications, which generate relatively little heat and have very long life cycles, carbon fiber may be fine if you can stomach the hefty price tag. Center fire barrels are another story. High-powered cartridges can heat a barrel in just a few shots. Trapping that heat near the bore will surely decrease the lifespan of the barrel, not to mention raise chamber temperature and pressure over lengthy shot strings.

To be fair, the addition of certain exotic media to carbon fiber epoxy resins can increase its thermal conductivity, but if barrel makers are doing so, they are mum about it. There’s an easy test to find out though. Put several shots in quick succession through your high-powered rifle. A barrel with good thermal conductivity (steel is half decent) will get warm to the touch very quickly, though of course the thickness of the barrel will have an influence. If your carbon wrapped barrel never gets noticeably warm, that’s a clear indication that the carbon fiber is insulating rather than transferring.

If your machine cannot incorporate dynamic cooling systems, like a modern engine with its water jackets, radiator, fan and pump, then material choices become very important to thermal performance. It may surprise many to learn that water is a lousy coolant. Its thermal conductivity is extremely low compared to most metals. Air is even worse. It’s actually a terrific insulator. But air and water are both cheap and plentiful, and if you throw enough of them at a hot spot, they will carry away the heat.

Your gun barrel does not have the luxury of a dynamic cooling system. Most of the time it sits in pretty static air, which is a better insulator than conductor. Air cooled engines had a pretty good run in the 20th century, and small ones are still used today, but they rely heavily on the rapid movement of air past cooling fins. Seen any cooling fins or fans on guns recently?

The firearms industry does not evolve at the breathtaking pace of the motorcycle industry. The most popular modern sporting rifle design is over fifty years old. The science of barrel making has improved, but still, the standard answer to the questions of barrel rigidity and thermal performance is: more steel. The concept has its limits. Steel gets heavy and cumbersome, and it’s hardly the ultimate thermal conductor anyway. Early machine guns used water jackets for cooling, and the Tommy gun had cooling fins near its chamber.Since then dynamic cooling has been all but abandoned on small arms.

Barrel steel is pretty cheap, but you aren’t reading this because you want to find out about the cheapest way to build a gun. There are plenty of cheap guns out there. It’s a race to the bottom, if you haven’t noticed.

High performance has a price tag. But the good news is, we’ve already eliminated two of the more exotic materials as candidates. A high-performance barrel doesn’t have to be made of un-obtainium. There are common materials with twice the thermal conductivity of steel. In Part II, we’ll talk about materials that make sense, and debunk some more hype. It’ll be fun. Bring your hot tubbing partner.