After years on opposite political battle lines, gun-control activists and the gun lobby are on the same side of at least one issue. Both the 2.5 million-member National Rifle Association (NRA) and the smaller but no less determined Handgun Control Inc. (HCI) are supporting legislation to restrict manufacture and use of ''cop-killer'' bullets.
Congressional passage of the measure, however, is by no means assured, despite substantial and broad-based backing within both the Senate and House and the endorsement of President Reagan.
Much could depend on to what extent Sen. James A. McClure (R) of Idaho succeeds in substituting a broader bill for the less controversial one, which involves only those types of bullets capable of penetrating the protective vests worn by police officers.
Efforts to speed Senate passage of the latter measure were thwarted June 28 when Senator McClure forced a delay on the bill. Some gun-control advocates such as Donald Fraher of HCI say the move could jeopardize the bill's prospects for approval this year.
The Idaho senator's bill, which the NRA also supports, appears to have little chance of getting through this year.
This measure would restrict neither the manufacture nor possession of armor-piercing bullets.
It deals only with their use in the commission of a crime, imposing a mandatory five-year prison sentence for first offense and 10 years without parole for second and subsequent convictions.
While similarly stopping short of banning ''cop-killer'' bullets outright, a Reagan-embraced measure, sponsored by Sen. Strom Thurmond (R) of South Carolina, seeks to restrict their manufacture and limit sales to police, US military forces, and foreign governments. And it would ban the importation of the bullets from other countries.
The legislation, which cleared the Senate Judiciary Committee June 15 and now has 90 cosponsors in the Senate, had been expected to race through that lawmaking chamber with flying colors before the current recess.
A similar bill in the House is being pushed by Reps. Mario Biaggi (D) of New York and Jack Brooks (D) of Texas is pending before the subcommittee on crime of the House Judiciary Committee, where it was aired June 27.
Several changes are being considered, most of them aimed at making the measure at least slightly tougher than the Senate version. One of these would involve an outright prohibition of sales of these armor-piercing bullets.
Representative Biaggi, a former New York City police officer who has been pushing such legislation for the past five years, wants to make sure that whatever is approved ''is not too weak to do the job,'' explains Craig Floyd, the congressman's legislative assistant.
Noting that this is the first time handgun-control forces and the NRA have been together on any legislation, Mr. Fraher of HCI says he hopes ''this will lead to future cooperation on other matters.''
He suggests this might include some type of restriction on ''snubbies,'' a type of short-barreled handgun sometimes known as ''Saturday-night specials,'' which are not used for either hunting or target practice.
The NRA, however, wants no part of anything that would restrict the ownership of such weapons, insists John Adkins, deputy director of education for the 113 -year-old organization.
Support for restrictions on the armor-piercing bullets, he emphasizes, ''does not mean any change in our position.''
His group has opposed such legislation in the past because it lacked a clear definition of what types of bullets would be restricted, Mr. Adkins explains, adding that the pending compromise measure has done this to his group's satisfaction.
''This is a recognition by the NRA that there are some types of bullets that have no useful purpose for sportsmen,'' Fraher says.
But Adkins makes it clear that his group could not go along with anything that would ban the sale of bullets or restrict their possession by law-abiding citizens.
Strong pressure for outlawing or restricting production and possession of armor-piercing bullets has come from police groups as well as gun-control forces throughout the United States.
President Reagan, members of whose administration worked on the compromise legislation that brought the gun lobby and gun-control advocates together, formally endorsed the legislation June 20 in a speech in Hartford, Conn., to the National Sheriffs' Association.