Alaska Science Forum

This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Larry Gednev is a seismologist who coordinated this column from 1982 to 1987, when he retired from the institute. This article was originally published in this column in 1989.

With every hunting season, handloaders renew the quest to squeeze that extra bit of velocity out of their pet load. The problem of projectile velocity haunts military minds as well as hunters' dreams, In the 1930s, it seemed as if the ultimate had arrived with the .220 Swift, which produced muzzle velocities of over 4,000 feet per second (fps). Modern big game hunters typically rely on weapons of larger caliber but lower velocity, about 3,000 fps. The standard in hunters' ammunition is still the old reliable .30/06. This venerable cartridge has a diameter of approximately .30 inches, giving its caliber, and was introduced in 1906, giving its suffix.

The "aught-six" fires a 180-grain bullet (one grain weighs one seven-thousandth of a pound) with a velocity of about 2,700 fps, a combination producing about 2,900 foot-pounds of energy. That's adequate for a pot-hunter sighting on a caribou at a reasonable distance, but it won't do for a tank commander sighting in an armored vehicle. The military projectile designers have to think big.

Tanks employ much larger tungsten-carbide projectiles driven at velocities in excess of 4,000 feet per second. At that velocity, the big projectiles literally melt their way through all but the newest, toughest armor.

Getting still more velocity is not easy. Chemical propellants cannot push a bullet beyond the speed at which the detonation wave propagates through the explosive and into the surrounding gases. Worse, though there are high-speed explosives such as Primacord or some plastiques with detonation velocities above 20,000 fps, there are physical barriers to driving explosively propelled projectile that set in at around 6,000 fps. You can stuff in all the propellant you want, and you'll not exceed that velocity a whit.

The military designers have set out to circumvent the limitations imposed by chemical propellants. In effect, they've traded a bang for a zap: they've come up with the Electromagnetic Gun (EMG).

In the March 1989 issue of The American Rifleman, Terry Metzgar reports that EMGs work like linear electric motors. Like an ordinary rifle, an EMG has a breech, barrel, and muzzle. Its bore is different; it has two parallel copper rails, one on each side. With a projectile at the breech between the rails, an electric current is applied. The current forms a magnetic field behind the bullet. As current flows between the rails, the moving magnetic field accelerates the bullet down the whole length of the barrel.

Using this technique, three independent laboratories have produced velocities beyond 26,000 fps. If somehow a 180-grain .30/06. bullet could be kept from disintegrating instantly when propelled at that velocity, it would achieve 270,000 foot-pounds of kinetic energy at the muzzle-equivalent to a ten-ton eighteen-wheeler traveling at 20 miles per hour.

Of course, something as light as a standard rifle bullet would merely splash on hitting a plate of tank armor. The military's problem is to send a heavy projectile at hypervelocities. They seem to be well underway toward doing just that: the Army and its contracting agencies recently tested a 90-millimeter EMG called "Miramar Gun B" that successfully shot a projectile weighing more than two pounds at a velocity of 11,154 feet per second. That yields a stupefying 4,300,000 foot-pounds of energy---the equivalent of that ten-ton semi traveling at 80 miles per hour.

A weapon with that kind of power should at least discourage an armored enemy force. It might be a little discouraging to the side that fires it as well---imagine the recoil! EMGs are not exactly a foot-slogger's weapon...at least, not yet.