A FEDERAL JUDGE here has just shot down a law that should have been killed long ago: the 55-year-old statute giving members of the National Rifle Association the exclusive right to buy surplus Army firearms at cut rates. This little prerequisite, part of the old program to promote civilian marksmanship, has been a very good deal for those who qualified.Though the Army's arms sales to civilians have dwindled since 1968, some NRA members this year may be able to buy surplus M1 rifles for $112; non-members would have to pay from $250 to $1,000 elsewhere for M1s. And the discounts have also been a very good deal for the NRA because they give civilian gun-buyers a strong incentive to join and thus help the organization boost its political strength.

It was the NRA's political stance that provoked the suit against the law. A Maryland man, backed by the National Coalition to Ban Handguns, complained that the provision was unconstitutional because it denied him equal treatment as a prospective gun-purchaser unless he joined a group with which he disagreed. Significantly, the government -- which usually defends federal laws against such challenges -- refused to do so in this case. Instead, the Justice Department and the Army told the court that the membership provision served no valid national purpose. District Court Judge Harold H. Greene agreed. NRA membership, he declared, does not mean that a person is a good marksman or a more desirable purchaser of surplus arms than a non-member active in target-shooting might be.

The Army, Judge Green said, has other, non-discriminatory ways to encourage civilian rifle practice if it wants to. He's right. But the Army is also right in concluding that the whole marksmanship program, with its annual target-shooting competitions and handouts of arms and ammunition to participants, is an anachronism that should be scrapped. So far, the NRA and its friends in Congress have managed to keep the program alive, although barely. This week's decision should help more legislators see whose interests the program really serves and why it should be stopped.