IT MAY SEEM odd that an obviously terrible piece of legislation -- S.49, the "killer bill" that would repeal important federal protections against quickie handgun sales -- has won significant support on Capitol Hill. Leaders and members of law enforcement groups throughout the country vigorously oppose it. But the biggest handgun pusher in America, the National Rifle Association, has done a bang-up job of buying support in Congress. With S.49 heading for consideration by a House subcommittee this week and possible floor action within two or three weeks, you can bet that the NRA's Political Action Committee will be calling in chits from its friends.

With the right amendments, S.49 could be turned into an acceptable bill. Attorney General Edwin Meese III and the Bureau of Alcohol, Tobacco and Firearms said as much, at least until the administration papered over their concerns with a blanket endorsement. Meanwhile the NRA's beneficiaries in the House have been signing a discharge petition to force S.49 to an up-or-down, as-is vote on the House floor. The petition is near the 218 necessary signatures. At last check, Handgun Control Inc., a group opposed to the measure, had found that 84 percent of 156 identified signers had received contributions from NRA/PAC in either 1983-84 or '85-'86. The list, which appeared in yesterday's editions of The Post, showed a wide range of contributions, including $1,000 to Virginia Rep. Stan Parris on up to $36,679 for J. Alex McMillan of North Carolina.

If S.49 isn't amended, the police, sheriffs and other law enforcement authorities who have to face the wrong ends of handguns want it killed. They object to a provision eliminating an existing federal prohibition against interstate sales of handguns. S.49 would allow over-the-counter sales by dealers in any state if the purchase complied with state laws in both the buyer's and dealer's states. This provision, coupled with the absence of any waiting period for handgun purchases, is an alarming invitation to "impulse buyers" and criminals.

If the interstate sales ban on handguns were preserved and a reasonable waiting period added, other changes could be made in S.49 to ease record-keeping requirements for dealers and to make it clear that Congress does not condone administrative harassment of dealers or sportsmen. Also, language should be added to extend a ban on the importation of Saturday Night Specials to include the parts for these cheap weapons of criminal favor. These are improvements that the House subcommittee should make to accommodate legitimate gun owners as well as the law enforcement authorities who must cope with violent crime.