"Cast and Recast: The Sculpture of Frederic Remington," which opens today at the National Museum of American Art, is an exhibit with a mission. Its side-by-side display of originals and fakes should make the bad guys squirm.

Frederic Remington (1861-1909), who had spent his New York childhood dreaming of the Wild West, set out in 1880 (after two years at Yale) to find the real thing. He rode with cowboys and with Indians, he was a ranch cook for a while, he prospected for gold, and ran a mule ranch in Kansas. "I saw the wild rivers and the vacant land were about to vanish forever," he wrote in 1905. "I saw the living, breathing end of three American centuries of smoke and dust and sweat." His illustrations of the West, his paintings and his drawings -- and especially his bronzes -- by then had made him famous.

One can hardly see a rodeo, or a Western on the tube, without recalling Remington. He, as much as anyone, set our image of the West. His sculptures are a treat. Their extraordinary details -- the sweat on that horse's flank, the worn heels of that cowboy's boots, the muzzle of that rifle, the rocks of that steep cliff -- somehow coexist with amazing, lifelike energy. These active, graceful objects are full of vim and vinegar. You can almost see them breathe.

The only trouble with these sculptures is that reproductions, replicas -- and rip-offs made by others long after his death -- today far outnumber the statues whose fine castings he supervised.

The present exhibition, organized by Michael Shapiro of Duke University, focuses on four of his best-known images -- "The Bronco Buster," "The Scalp," "The Cheyenne" and "The Mountain Man." Remington was a perfectionist. You need not be a connoisseur to see how good his own works are -- or how crummy are the rest.

The visitor may here compare 10 quite different versions of "The Bronco Buster," Remington's first bronze.

The first of these was cast -- in sand molds -- by the Henry-Bonnard Bronze Co. in 1895. Various pieces of the sculpture -- the right arm of the rider, his upper torso, and his stirrups, and the tail of the horse -- were separately cast and then joined together. The Henry-Bonnard Co. eventually produced perhaps 70 "Bronco Busters," all of which were made using the same method. They are pretty much alike.

Then, in 1900, Remington met Riccardo Bertelli, a Genoa-born technician who introduced the sculptor -- and America as well -- to the far more flexible and subtle lost-wax method. The "Bronco Busters" made at Bertelli's Roman Bronze Works were cast in a single piece. Their surfaces were more finely detailed. And because each casting required a new wax positive, Remington, for the first time, could alter little details between one cast and another. By touching brush to wax, for instance, he could indicate the hair on the rearing bronco's leg. He made other changes, too. Fiddling, refining, he changed the horse's eyes and mane, the right hand of the rider, the position of his stirrups, and texture of his chaps (once they had been leather; Remington changed that, in a few casts, to thick, fleecy wool).

Some 90 "Bronco Busters" were made by the Roman Bronze Works before the artist died. His widow then authorized the casting of a posthumous edition of 200 more. Though her will stipulated that "all bronzes done by my late husband Frederic Remington must cease being produced after my death," that was not to be. Spurious "Bronco Busters," made with rubber molds from an "original" (or possibly a fake), are still being cast today. Because "The Bronco Buster's" copyright expired in the 1970s, these recasts aren't illegal, but they sure are ugly.

In 1978, sculptor Cecil Golding made a three-quarter-size "replica" of the statue -- in an edition of 1,000. "The low point in contemporary reduction and replication of Remington's sculpture," writes Shapiro, "is found in the paperweight-size replicas" made by sculptor Dale Weston in 1979. One is on display. Fans of Western art who've been snookered into thinking there is no big difference between "originals" and "recasts" ought to see this show.

One reason for the blurring and gradual corruption of "The Bronco Buster" here is the still-growing demand for works of Western art. Another, of course, is plain, old-fashioned greed. A good Henry-Bonnard sand cast of the piece sold Wednesday in New York for $110,000.

Remington, who took enormous pains with the smallest details of his lifelike statues, would have loaded up his six-gun at the infuriating sight of the rip-offs and recastings included in this show. This scholarly exhibit performs a useful service. It closes Jan. 3.