Page:Popular Science Monthly Volume 91.djvu/770

 Powder Gases Wash Away Steel Guns

The process is the same as that by which a river cuts away its banks

By Edward C. Grossman

��WHEN a modern infantry-rifle cart- ridge is fired in a rifle-barrel, the heat generated by the powder gases is measured by some 4000 degrees Fahren- heit. One shot does not mattermuch, two shots in rapid succes- sion raise the temperature of this steel a bit more, three shots are still worse, but. twenty, fired at machine- gun speed, raise an or- dinary bar- rel to a steak- broiling tem- perature.

The gases pushing be- hind the bul- let and trav- eling at high speed with the pressure of 50,000 pounds to the square inch, literally wash away the softened steel just ahead of the chamber as fast as it is heated by the high temperature of dis- charge. The process is known as erosion — a washing away. When a river sets in to cut away its banks, it also erodes.

Because machine-guns are fired at high speed and are used for a comparatively long series of shots without a pause, their chief foe is this erosion. If machine-guns were fired as the average layman supposes — one continuous blast of bullets while the argu- ment lasts, they would be out of commission in a couple of thousand shots, which means an actual firing life measured by a few minutes. No cooling system ever invented

���These are American machine guns which have been nreu irom 2,000 to 3,000 times. Ahead of the chamber, which is the por- tion where the cartridge rests when the gun is loaded, the barrels are washed away on either side until the bore is larger than the neck of the chamber. The result is that the hot gases rush past the bullet, which does not seal the gap, and wash still more steel away, and so on, around the circle

��will keep down the temperature on the in- side of the barrel with such usage.

Because of this, because of the necessity for re-laying the gun on the mark every

twenty shots or so, and because of the slight de- lay in insert- ing a new belt or drum or clip of arrmunition, the guns are fired in short bursts, each followed by a pause. The Germans use two guns at detached points, alter- nating their fire to avoid over-heating and to avoid the delays incurred by inserting new clips or belts.

Even used in this way, machine guns show erosion very quickly. The photograph shows the barrels of types of American machine guns, fired 2,000 and 3,000 shots. As the photographs plainly show, ahead of the chamber, which is the portion where the cartridge rests when the gun is loaded, the barrels are washed away on either side until the bore is larger than the neck of the chamber.

The result is that the hot gases rush past the bullet which does not seal the gap, and wash still more steel away, and so on, around the vicious circle of damage. The barrels shown in the photographs are sectioned lengthwise, showing only one half of the bore of the guns.

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