The National Rifle Association and gun-control advocates are in rare agreement about one aspect of a new federal restriction on rifles and shotguns - few people know about it.

Starting Dec. 1, deer hunters and other sportsmen will have to pass an instant background check before they can buy a long-barreled gun.The background check, done on the spot in the gun store, will be the first national screening of hunters, target shooters and others buying rifles and shotguns.

Handgun Control Inc. President Bob Walker said the inclusion of long guns drew little notice when Congress passed the Brady law in late 1993 and even less since then.

"There appear to be quite a few people that have either forgotten about this or never paid any attention in the first place," agreed Tanya Metaksa, the NRA's chief lobbyist.

Buyers of "long guns" were exempt from the Brady Act's requirement of a background check and five-day waiting period for handgun purchases. The law took effect in early 1994.

But that compromise legislation also looked ahead five years to a day when an instant, national computerized background check system would be in place, eliminating the need for the five-day waiting period and allowing the expansion of background checks to all gun purchases from registered dealers. That day is Dec. 1.

Both sides in the perennial argument over gun control are girding for friction as gun buyers confront the new restriction. Gun merchants, who must explain the new system to prospective buyers, predict startled and angry customers.

"Unless they are pretty active in gun issues or very well-read, I bet you the overwhelming majority of gun owners are totally unaware this is going to kick in," said Steve Schneider, owner of Atlantic Guns in Silver Spring, Md.

Joe Garcia, looking over the shotguns at a Wal-Mart near Waldorf, Md., knew the law will change but was unclear about the details.

"They're trying to take away the people's freedom by pieces," he complained.

Until now, there was no comprehensive check on whether long-gun buyers were telling the truth on federal firearms forms. The forms ask buyers whether they are felons or otherwise barred by law from owning guns.

Eight states - Illinois, Massachusetts, New Jersey, Virginia, Florida, Pennsylvania, Connecticut and Delaware - already have some form of background check covering long guns, according to Handgun Control, which led the fight for the Brady Act on Capitol Hill.

The issue is tricky for the NRA, which helped engineer the switch from a waiting period to the instant check system when the Brady bill was before Congress. The NRA regards the instant check as less onerous, since the vast majority of buyers will be able to walk out of the store with their guns instead of returning later to pick them up.

The instant background check system will be much the same as the instant checks now done in many states on would-be handgun purchasers.

It will scan criminal and mental health records, military discharge rosters and other databases to see whether a customer may legally buy a gun. The system will not reveal why a customer has been rejected.

If it rejects a would-be buyer, it will put a "hold" on the sale, giving the store and customer three days to assess the problem. If the computer rejects the buyer again, there is an appeal process.