A Montgomery County judge rejected last-minute efforts yesterday by gun store owners and gun enthusiasts to stop implementation of a law banning ammunition sales to people without proof of gun registration.

Circuit Judge Stanley Frosh scheduled a hearing on the injunction request for Oct. 20, but affected gun shop owners said they will lose thousands of dollars in business in the meantime. The bullet-ban legislation applies to only four stores in Montgomery County, according to those seeking the court order, because it exempts incorporated municipalities, in which at least half a dozen gun shops are located.

The law, passed by the County Council in June, and which went into effect yesterday, requires persons seeking to buy ammuniton for handguns to produce proof that their weapons are registered.

Under state law, all handguns must be registered when they are bought at a gun store, but subsequent resale and private transactions require no registration. No registration is required for rifles and shotguns.

County Council President David Scull said yesterday that the law "indirectly encourages the vast majority of law-abiding gun owners to do something of proven value to the police--namely, to register their guns."

Stephen G. Schneider, vice president of Atlantic Guns Inc. in Silver Spring, one of four persons seeking the injunction, said yesterday that the law will do nothing to encourage registration because gun owners can shop in any of the county's municipalities. Schneider estimated in court papers that his business will lose $4,000 a week in ammunition and related sales.

Ron Poling, manager of Crawford International, a Silver Spring gun shop that is not involved in the suit, complained that a customer that he must turn away "can go to Kensington, Rockville, Gaithersburg or Prince George's County and buy all the ammunition that he wants."

Poling also said that much of the ammunition used for rifles also can be used in handguns, so it also is affected by the law. Furthermore, he said, persons who have registered guns usually don't keep a copy of the registration because it is not mandatory.

By late yesterday afternoon, Poling said that of about 15 people who tried to buy ammunition, only one was able to produce proof of registration. The rest were turned away, he said.

Another person seeking the injunction was William H. Randolph, a retired Gaithersburg man and vice president of Montgomery Citizens for Just Firearms Legislation. Randolph, a collector of antique guns who also enjoys shooting, said he is worried that registering his guns will make him an easy target for thieves. "You're going to be in big trouble," he said. "You are going to have a lot of gun thefts."

The main issue in the court battle is likely to be whether the county law contradicts a 1957 state law that says regulation of the sale and carrying of handguns is a state rather than local matter. Both the county attorney and Maryland's attorney general have said they consider at least some versions of the Montgomery County law contrary to state law.

As a result, the council has hired a private Rockville attorney to represent it in court. That lawyer, Roger Titus, represented the village council of Friendship Heights in an unsuccessful effort last year to ban bullets.