Block that Kick!: Quick‑Firing Field Pieces

 

 

National Archives from Army

Howitzer shelling Germans in 1944 recoils after firing a shot.

 

It really wasn’t like the movies. In films about the American Revolution or the American Civil War, for example, the muzzle‑loading cannons fire, throwing out some smoke, and the gunners, who have been standing beside them and behind them, immediately reload and fire again. Actually, the guns of those days threw out a lot more smoke, because the film‑makers don’t use full charges of black powder. And the real gunners didn’t stand behind the cannons and reload as soon as they fired. The recoil of the shot blew the gun back quite a few feet, and standing behind a heavy cannon when it fired was a good way to keep from growing old. The movie cannons don’t recoil, because they really don’t fire shot or shell. Property‑owners in the vicinity would take a dim view of cannonballs holing their roofs or shells exploding in their gardens.

Before the cannon could be fired again, it had to be swabbed out with a wet “sponge” (actually a wad of wool on the end of a ramrod) to kill any sparks that might be in the gun. Otherwise, the powder charge might ignite and drive the ramrod through the gunner before he had a chance to load the shell or cannonball. Swabbing took some time. Even more time‑consuming was the need to realign the gun. The gun crew had to manhandle the cannon back to its original position and aim it again. Even a comparatively light gun such as the 12‑pounder “Napoleon” of the Civil War weighed more than a ton, and aiming the gun usually involved lifting the trail of the heavy carriage to swing the gun around.

By the Civil War, another problem had appeared. Rifles had become so accurate at long range that using artillery at traditional ranges had become almost suicidal. Artillery could no longer be used in the front line with the infantry. The gunners stood in the open, and there was nothing to give them protection from enemy rifle fire. Recoil made it impossible to hide behind the gun.

The latter part of the 19th century was a period of tremendous progress in artillery design. One prime objective was to increase the effective range of cannons. Another was to increase their speed of fire. Achieving both of these objectives meant overcoming recoil.

One way to increase the range was to fire guns at a higher elevation. Most of the cannons until this time were what artillerymen technically call guns – comparatively heavy, long‑barreled weapons that have a higher muzzle velocity than the shorter barreled howitzers and mortars and fire their projectiles on a flatter trajectory. If a gun could be elevated to fire on a higher trajectory, its projectiles would go farther. But more of the recoil would be directed down at the carriage. That proved to be too much strain on the old wooden carriages. Gun makers switched to metal carriages, particularly steel carriages. Steel, much stronger than bronze, cast iron, or wrought iron was just starting to become available in large quantities at reasonable prices. Krupp pioneered making gun barrels from steel. Steel barrels could handle heavier powder charges, which also increased range.

Rifling a cannon barrel also increased its effective range. The main problem was getting a hard iron or steel shell or solid shot to “take” the rifling. Various methods were tried, including casting lugs on the shell that would fit in the grooves of the rifling. The method finally adopted was surrounding the projectile with a band of softer metal, usually a copper alloy, that the “lands,” the raised portions of the rifling, could bite into. That, and another soft metal band that rode above the lands, sealed the bore so that none of the expanding gas from the explosion of the propelling powder charge leaked around the projectile. Before that, muzzle‑loading cannons had to allow for “windage.” The projectile had to be smaller than the bore. On the 12‑pounder Napoleon that difference came to.01 inches. The absence of windage increased both the range and accuracy of the cannon. So did the fact that rifling made a projectile travel nose‑first. Rifled guns could use an elongated projectile, one much heavier for its diameter than a round ball. This increase in “sectional density” meant that an elongated projectile had far more range than a round one with the same muzzle velocity.

It’s much easier to load a rifled cannon from the breech than from the muzzle. The use of steel and improved breech blocks made breech‑loading so attractive that muzzle loading practically disappeared except for small trench mortars. Two types of breech block were used. One was a sliding block of steel; the other resembled a small, extremely thick bank vault door that was locked by an interrupted screw surrounding it.

There were three types of ammunition for breech‑loading cannons – fixed, semi‑fixed and bagged. Fixed ammunition resembled a rifle cartridge, with the shell fitted into a brass cartridge case containing the propelling charge. Semi‑fixed also had a brass cartridge case, but the powder charge, packed in bags, could be varied to vary the range. With bagged ammunition, the shell was loaded first then the powder charge in one or more bags. Fixed ammunition and semi‑fixed ammunition depend on the brass cartridge case expanding when the propelling charge is ignited. That seals the breech against escaping gas. To use bagged ammunition, the interrupted screw breech block has a “obturation pad” on its inner face that expands when the propelling charge explodes. To use the sliding breech block with a bagged charge, a gunner inserts a separate copper sealing ring behind the bagged propelling charge.

Steel, breech‑loading, rifled guns were a huge step forward. They had more range and far more accuracy than their predecessors. But when makers like Hotchkiss and Krupp advertised their quick‑firing field pieces they were using a bit of hyperbole. They still hadn’t dealt with that old devil, recoil. After a gun fired, the gunners had to push it back into position and aim it once more. Only if someone found a way to keep the gun in position and on‑target during firing, could the cannon be truly said to be quick‑firing.

Inventors came up with a variety of systems to hold the gun in place. On sailing ships, guns were allowed to roll back a certain distance, then the motion was stopped by a thick rope attached to the gun. Gun crews then used other ropes and pulleys to haul the cannon back to its gunport. If the restraining rope broke, however, you would have the proverbial “loose cannon on the deck” – a most undesirable situation. In some fortresses, guns were allowed to roll up a steep ramp. Gravity then repositioned them. One ingenious device, also used in fixed fortifications, was the disappearing gun. The gun was in a concrete‑lined pit below the surface of the earth. Machinery raised it to firing position with the aid of counterweights. When the gun fired, the recoil returned it to its pit. An enemy would have only a brief glimpse of the gun before it fired and disappeared. Airplanes made the disappearing guns obsolete, but they were still used in U.S. coastal defenses in World War II.

Recoil was easier to treat in forts than in the field. Some designs attempted to absorb recoil with rubber buffers, but rubber wore out quickly, froze in cold weather, and wasn’t all that effective at any time. The 15‑pounder field piece the British used in the Second Boer War had a “recoil spade” attached to the carriage axis. The spade was attached to a steel spring fixed to the carriage trail.

Gunners dug the spade into the ground. When the gun fired, the whole gun and carriage rolled back, but the spring drew it back to position – more or less. It sounds better than it worked. The spade itself did not stay immobile. The recoil pulled back the arm to which the spade was attached, changing its angle to the ground, so the gun never returned to exactly the same spot.

The first field gun to solve the recoil problem was the French 75 mm Model 1897. A retired officer, Commandant de Port, modified a German invention to produce the system upon which all modern recoil systems are based. The “French 75,” as American World War I veterans called it, had a barrel that could slide back but that was attached to a piston in an oil‑filled cylinder. When the gun barrel recoiled, the piston pushed the oil through a small orifice and into a second cylinder. That oil pushed back a floating piston in a second cylinder, compressing the air in that cylinder. Squeezing the oil out of the first cylinder absorbed much of the energy of the recoil; compressing the air in the second cylinder took care of the rest. Air is an extremely elastic material. When the gun’s motion stopped, the compressed air reasserted itself, bringing the gun back into firing position. The trail of the gun carriage had a spade that was planted in the ground to keep the gun carriage from moving. The gun was ready to fire another shot immediately. Because the gun carriage didn’t move at all, it was possible to hang a bullet‑proof shield on the gun. That was a great boon to gunners. It also made possible the U.S. World War II experiment of attaching “cannon companies” to the infantry. The cannoneers worked right up with the riflemen, providing close‑in support with their 105 mm howitzers. That was dangerous work, to be sure. But without the shield, it would have been suicide.

The recoil mechanism also made it much easier to dig in artillery pieces, a practice that was common in both world wars, Korea, and Vietnam. Without something to absorb their recoil, guns of the power of those used in modern wars would have to roll back a long way, so digging them in would require an enormous pit.

The French 75 used fixed ammunition. When a gunner opened the breech, the brass cartridge case was automatically ejected, and another round could be loaded. A trained crew could fire 30 shots a minute from the 75 – faster than most infantrymen could fire a bolt‑action rifle. The M1897 75 mm was the standard French and American light artillery piece all through World War I and for many years afterwards. The French were still using it in World War II.

Every field gun in the world and most of the naval guns and the big siege guns copied the recoil system introduced on the French 75. As a result of the speed of fire it made possible, artillery was far and away the greatest killer of all guns used in World War II. Artillery and mortars killed two thirds of all the soldiers who died in that war. Speed of fire was especially important to antiaircraft guns. German antiaircraft fire during World War II probably shot down more Allied planes than German fighter planes, and in Vietnam antiaircraft guns destroyed 91 percent of all American planes lost in combat.

 








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