Sleek, its lines rakishly tilted to boost the ergonomics that index grip placement to barrel, this automatic pistol has but one function: to eliminate human beings easily. That sinister intent is expressed most eloquently in the extended magazine that reaches far beneath the pistol grip, easily tripling the amount of ammunition available to the killer.

It's the Colt Super .38 automatic pistol, customized into a machine pistol by an underworld gunsmith so that Babyface Nelson could use it to kill an FBI agent outside Little Bohemia, Wis., in 1934. Maybe you saw the movie.

Even if you didn't, you can still see the point: There's nothing really new when it comes to guns. To the contrary, the extended magazine that Jared Loughner allegedly carried in his Glock 19 the day he is accused of having fatally shot six people outside Tucson and wounding 13 others, and that President Obama is likely to suggest banning in an upcoming speech, may be traced way back.

During World War I, American armorers tried to adapt the 1903 Springfield into a counter-sniper "periscope rifle" by, among other things, installing a 25-round magazine. The Germans tried to turn the Luger pistol into a "trench broom" by devising a 32-round "snail drum" magazine (it fired the same round as the Glock 19). The Texas Ranger Frank Hamer carried a Remington Model 8 with an extended magazine in his hunt for Bonnie and Clyde in 1934. The Thompson submachine gun of World War II and the M-16 of Vietnam were improved by extending their magazine from 20 to 30 rounds. In 1957, the U.S. Army adopted the M-14 rifle, which was hardly more than an M-1 Garand rifle with an extended magazine. And who wouldn't want our soldiers, Marines and law officers to benefit from extended magazines?

Guns were the software of the 19th century; the most dynamic age of development was roughly 1870 to 1900, when the modern forms were perfected. Two primary operating systems emerged for handguns: the revolver, usually holding six cartridges and manipulated by the muscle energy of the hand, and the semiautomatic, harnessing the explosively released energy of the burning powder to cock and reload itself. Since then, design and engineering improvements have been not to lethality but to ease of maintenance and manufacture, or weight reduction. A Glock is "better" than a Luger because you don't need a PhD to take it apart, nor a fleet of machinists to produce the myriad pins, levers, springs and chunks of steel that make it go bang. Moreover, you can lose a Glock in a flood and find it six months later in the mud, and it still will shoot perfectly, while the Luger would have become a nice paperweight.

What nobody has been able to improve on since the 1870s is the cartridge. It is an extraordinary mechanism that safely stores volatile chemical energy until needed. It is cheap to manufacture, easy to transport and largely impervious to the elements.

What's often lost amid activists' carping is that the effect of the notorious extended magazine does little to improve the pistol's lethality except in extraordinary circumstances, such as Tucson. Neither Seung-Hui Cho, the Virginia Tech mass murderer, nor the alleged Fort Hood killer used extended magazines in their rampages. America's first gun mass murder, when Howard Unruh killed 13 people in 1949, was committed with a Luger.

In fact, the extended magazine actually vitiates the pistol's usefulness as a weapon for most needs, legitimate or illegitimate. The magazine destroys the pistol's essence; it is no longer concealable. Loughner allegedly wrapped the clumsy package in a coat for a short distance, but he could not have worn it in a belt or concealed it for an extended period. It had really ceased to be a pistol.

That's why extended magazines are rarely featured in crime - and that awkwardness spells out the magazine's primary legitimate usage. It may have some utility for competitive shooting by cutting down on reloading time, or for tactical police officers on raids, but for those who are not hard-core gun folks it's an ideal solution for home defense, which is probably why hundreds of thousands of Glocks have been sold in this country.

Particularly in rural Arizona, given the upsurge in border violence, it's likely that residents feel the need to defend themselves against drug predators, coyote gunmen or others. Yes, they can use semiautomatic rifles and shotguns, protected by the Second Amendment and unlikely to be banned by local law, but women generally don't care to put in the training needed to master them. Nor can the elderly handle them adeptly.

For them, the Glock with a 33-round magazine is the weapon of maximum utility. You can load it on Sunday and shoot it all month. (Nobody wants to reload a gun while being shot at.) It's light and easy to control. You don't have to carry it or conceal it; it's under the bed or in the drawer until needed. When the question arises of who needs an extended magazine, the answer is: the most defenseless of the defenseless.

Those who would ban extended magazines, will say that although hundreds of thousands are in circulation and thousands more will surely be sold before a ban is enacted, it will be worth it if it saves just one life. But the other half of that question must be asked, too: Is it worth it if it costs just one life?

Stephen Hunter, a former chief film critic of The Post, is the author of "Dead Zero."