WHEN THOUSANDS of America's law enforcement officials are vigorously opposed to S.49 -- a killer bill that would gut key federal protections against quickie handgun sales and traffic in this country -- it's little wonder that some senior officials in the Justice and Treasury department have serious reservations about the measure. They should. But they're not getting through to their own administration. Even though Attorney General Edwin Meese III and Treasury's Bureau of Alcohol, Tobacco and Firearms have voiced concerns about the effects of the bill on efforts to battle crime, their objections were papered over in the administration line presented to a Senate subcommittee last week.

At the Senate hearing -- with the Justice Department conspicuously unrepresented -- Deputy Assistant Treasury Secretary Edward T. Stevenson denied any split or change in position, saying that "Justice, Treasury and the White House are still consistent in support of" S.49. That's music to the ears of the lobbyists for the National Rifle -- make that read handgun -- Association, but it's a blow to an earlier Reagan administration recommendation. Former Attorney General William French Smith's 1981 Task Force on Violent Crime recommended a 14-day waiting period for purchases of handguns, to facilitate basic record checks because "drug addicts, felons, mental defectives and the like are not the best risk for the 'honor system.'is and other attempts to assist law enforcement authorities when it approved S.49.

Now it's up to members of the House to preserve existing protections by rejecting S.49. Doing so would be in the interest of sportsmen, gun collectors and anybody else who understands the difference between legitimate purchases of firearms for recreation, law enforcement and private security, and quickie sales of concealable handguns to criminals and "impulse" purchasers. Americans who do understand the difference -- and value existing protections -- will be looking to the House for help.