Firing a Cannon Like a Rifle: Recoilless Guns

^{National Archives from Army}

^{U.S. troops fire 75 mm recoilless gun at North Koreans in 1951.}

In the armies of Napoleon and Gustavus Adolphus, cannoneers and their guns fought right up in the front lines as the infantry. That had many advantages. Front line commanders didn’t have any trouble getting fire support from the artillery. But when rifles were adopted, standing up in the front line loading a cannon meant that you probably would not get a chance to tell your grand‑children any war stories.

There were some attempts to rectify the situation. Until World War II, the most successful was the trench mortar. In the First World War, French and American infantry troops used a light, low‑power 37 mm gun. It was good for knocking out machine gun nests, but very little else. In the Second World War, the American Army experimented with “cannon companies,” artillerymen who dragged a 105 mm howitzer up to the front and used it to give direct fire support. The trouble was that the gun was a magnet for enemy fire, and life in a cannon company tended to be short.

The greatest disadvantages of modern artillery pieces is their weight and bulk. The carriage has to be massive and heavy to withstand the stress of recoil, even though the guns are equipped with a recoil‑absorbing mechanism, which artillerymen call a recuperater. The recuperator also adds weight and bulk. If recoil could be eliminated, the gun could be smaller and lighter. The first man to eliminate recoil from a cannon was a U.S. Navy officer – a Commander Davis. A gun recoils because, as Isaac Newton stated, every action has an opposite and equal reaction. A shell is much lighter than the gun that fires it, so it travels at high speed and goes a great distance. The gun does not recoil at the same speed and, even without a recuperater, it doesn’t travel anything like the distance the shell goes. If the gun fired a missile of the same weight from each end of the barrel, there would be no recoil at all. That’s what Davis did. The missile from the rear end of his gun was mixture of lead shot and grease so, unlike the shell fired from the front end, it quickly dispersed. Davis sold his gun to the British, who used some of them on naval aircraft during World War I.

Recoil depends on the mass of the missile being fired times its velocity. If the Davis gun could fire a rear missile weighing half the “business” missile but at twice the velocity, recoil would still be eliminated. In calculating recoil, you have to figure the mass of the gas, as well as the mass of the missile. The mass of the gas is roughly the same as the weight of the powder charge. Some of the powder does not leave the gun in the form of gas but remains as residue. With smokeless powder, however, this is negligible, so it would be possible to eliminate recoil by ejecting only gas from the rear of the gun, provided the gas could be ejected at a high enough velocity to balance the force of the shell being fired.

Because it’s gas that’s ejected, the danger zone behind the gun is much shorter than if any kind of solid were ejected. Still, there is a fan‑shaped danger zone behind these guns that may extend more than 100 feet. The gas jet also kicks up a huge cloud of dust, which makes it easy for enemies to locate the gun.

The first to put this principle to practical use was the German firm, Krupp.

In 1940, Krupp produced a 75 mm light gun for airborne troops. The gun used fixed ammunition, but the base of the cartridge case was plastic. When fired, the plastic shattered and blew out a hole in the breechblock. The hole was a venturi, a tube with a narrow center section and widened, tapered ends designed to increase the speed of gas ejected through it. That speed was carefully calculated to equal the action of the shell being fired from the muzzle. The light gun had a carriage of light alloy and motorcycle wheels. It weighed only 321 pounds, compared to 1.1 tons for the regular 75 mm field gun, but had velocity and range only a little less than that of the regular gun. The Germans used the light gun during their invasion of Crete, and it was such a success they ordered two more recoilless guns, a 105 mm and a 150 mm.

The British also produced a recoilless gun design, invented by Sir Dennis Burney. The biggest difference between the Burney and Krupp guns were the ammunition they used. The Burney gun had a cartridge case with a few large holes punched in it. These were covered by thin brass sheets that blew out when the gun was fired. The escaping gas traveled to the rear around the cartridge case and was ejected from several venturis. Burney also invented a projectile for his gun, something he called a “wallbuster,” intended for use against fortifications. The wallbuster developed into the HESH or HEP shell (see Chapter 39) and turned out to be a good antitank round. It is less effective with modern layered armor, because that type does not transmit shockwaves through the metal so well, and the explosion of a HEP shell is less likely to break off significant “scabs” of metal.

Neutral Sweden got into recoilless gun design early, bringing out a 20 mm antitank gun in 1942. It used fixed ammunition with a plastic base cartridge case similar to the Krupp gun. It followed this rather ineffectual weapon with a much more formidable 105 mm gun.

The United States developed a different recoilless gun. As the others did, it used fixed ammunition, but the cartridge case was punctured with many small holes, instead of a few big ones, as in the Burney gun. It was nicknamed the Kromuskit from the names of its designers, Kroger and Musser. The shell’s driving band was pre‑engraved to fit the gun’s rifling. That meant that less gas pressure was needed to send the shell on its way at a decent velocity, and that meant that the barrel of the gun could be lighter. Also, a larger proportion of the propelling charge would actually be pushing the shell. Earlier recoilless guns needed a powder charge five times heavier than used in a standard gun – most of the burning gasses being ejected through the venturi rather than pushing the shell.

The first Kromuskit was a 57 mm gun weighing only 35 pounds. It could easily be fired from one man’s shoulder. The next one, a 75 mm, weighed 115 pounds – a bit heavy for shoulder firing, but usable on a machine gun tripod.

All of these recoilless guns fired shaped charge as well as ordinary high explosive shells. The Kromuskit guns also fired white phosphorus shells, which were both antipersonnel and smoke shells, and canister shot, which turned them into giant shotguns for use against personnel at close range.

Most of the recoilless guns lost some of the efficiency of their shaped charge antitank shells because they were rifled. Spinning decreases the power of the jet blast of a shaped‑charge explosion. The Swedes avoided that trouble with their 84 mm Carl Gustaf recoilless gun. The shell is fitted with rotating bearings. The rifling spins the bearings, imparting enough gyroscopic stability to keep the projectile on course, but the core, containing the shaped charge, does not spin.

At 38 pounds, the Carl Gustaf is light enough to fire from the shoulder and is able to penetrate 15.75 inches of homogeneous armor at a range of 500 yards. A second Swedish recoilless gun, called the Miniman, is disposable. Fire the one shell packed in it and throw it away. It’s a smoothbore, firing a shell stabilized by tail fins, has a range of 250 yards and can penetrate 11.8 inches of homogenous armor. It weighs only 6.31 pounds.

Germany also has a disposable recoilless gun. Like the Swedish model, it’s a smoothbore firing a finned shell. But no gas escapes from either the muzzle or the rear end. The propelling charge moves two pistons to the front and to the rear. The front piston throws out the shell and the rear pistol ejects a solid counterweight that is designed to rapidly disperse. Presumably, this makes the gun less visible on firing than the traditional recoilless gun. The German gun, called the Armbrust, is also light enough to fire from the shoulder.

Recoilless guns give the infantry direct fire artillery for the first time in centuries. They are available for any job that calls for something heavier than rifle or machine gun fire. Especially, they are available for antitank and – although the situation has not yet occurred – anti‑helicopter work.