NEW YORK (AP) _ While the wife of White House press secretary James Brady testified a cooling-off period for handgun purchases might have stopped John Hinckley Jr., the president of a pro-gun group said gun control laws do not deter criminals.

''Waiting periods will not stop the John Hinckleys,'' said Lawrence D. Pratt, executive director of Gun Owners of America, at a House Judiciary Committee hearing here Monday.

''He bought two guns in California in spite of a 15-day waiting period. Privacy laws then, and would now, prevent authorities from getting access to the psychiatric records,'' said Pratt, a supporter of the McClure-Volkmer bill, which opponents say would weaken the 1968 Gun Control Act. '' Criminals pay no attention to the legal purchase requirements.''

Among those testifying against the bill, which the Senate has already passed, were Sarah Brady, crime victims, law enforcement officials, religious leaders and a pediatrician.

''There have been many times that I have found myself thinking, 'Why is it possible for the John Hinckleys of this world to walk into a store, buy a handgun and go out and shoot people because they hear voices or have strange visions?''' testified Mrs. Brady.

''Had a waiting period and background check for handgun purchasers been in effect, John Hinckley could have been stopped,'' she continued in her first public appearance to speak about her husband since he and President Reagan were shot by Hinckley on March 30, 1981.

The bill is backed by pro-gun groups who say the 1968 act has failed to reduce violent crime, and its provisions are onerous and in some cases unconstitutional.

Seven gun control opponents were to testify in favor of the bill, but three left the hearing and joined Roy Innis, the president of the Congress of Racial Equality who was turned away from testifying, in denouncing the proceedings as a sham.

Police officials testified the bill would allow people with misdemeanor convictions to purchase handguns, make tracing gun ownership more difficult and delete the current provision permitting federal officials to conduct unannounced inspections of records maintained by federally licensed gun dealers.

The bill would allow one inspection per year with advance notice.

''In an age in which most homicides of police officers are committed with handguns and approximately 9,000 homicides annually are committed by guns in this nation, we should be working to eliminate guns from our society,'' testified New York City Police Commissioner Benjamin Ward.

''More startling is that the legislation would amend the mandatory prison sentence imposed for possession of a firearm during the commission of a violent felony,'' Ward said. ''The prosecutor would have the additional burden of establishing the firearm was carried 'in furtherance of' the felony.'''

Rep. Bill Hughes, D-N.J., said Innis was one of many people who had to be turned away Monday. He said Innis was told he would be invited to testify before the committee in Washington.

Innis said ''the reason for us being censored is it would bring a different point of view. The only people who'll be regulated by tougher gun laws will be decent citizens.''