HOW CARELESS of us the other day to omit mention of a critical element in the handgun industry's spring line of deadly items for sale at your nearby dealer: the exploding "Devastator" bullet, especially designed to blow up and fragment on impact. But only now do we learn to our horror that these super-charged cartridges were used in the shootings of President Reagan and Messrs. Brady, McCarthy and Delahanty. What if all the grisly gimmickry had performed as advertised? And what are those things doing on the open market, anyway?

This latest jarring development does contribute to a better appreciation of the shopworn "guns-don't-kill-people" slogan -- because when you think about it, guns really don't kill people very well without bullets. And the more powerful the bullet, the better the job a gun should do. This, we assume, is the rationale of the makers of the "Devastator," who came up with an aluminum cap filled with explosive that can be put in the nose of a normal .22-caliber cartridge. The idea is effectively illustrated in an advertisement showing a picture of a shattered cola can that has been shot at from 50 feet away.

Shooting cans of course is not the use of bullets that so concerns a growing number of Americans. There are other uses cited by officials of Bingham Ltd., the firm in Norcross, Ga., that manufactures the "Devastator"; for example, the bullets have been carried by federal sky marshals on planes. But why should any crazed person with $7.12 for a box of a dozen of these extra-lethal bullets be able to walk into a shop and stock up?

The question for every member of the House and Senate, then, is whether handguns and high-powered ammunition belong on the open market -- and the answer should be obvious.