Superman not only used a gun in SUPERMAN #8,
he used one twice in the same story. The second time he used a gun as Clark Kent, to extract a confession from one of the bad guys.
Not only that, but in ACTION COMICS #32 Superman invented and used a "Krypto-Ray Gun" which photographed evidence to later be projected for the authorities. The Krypto-Ray Gun was then marketed as a kid's toy with a selection of Superman filmstrips which kids could project on the wall, much like the later "Give-A-Show Projector".
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The Krypto-Ray Gun was supposed to be made of "Kryptonite". Strangely enough, this seems to have been the first time they actually used the word "Kryptonite", which would not be used in the comics until years later.
A story with kryptonite was actually slated for SUPERMAN #8, but was rejected by the editors, at least partly because it had Superman revealing his secret identity to Lois Lane during the course of the story. But in this story, the term "K-metal" ( for Krypton metal ) was used, rather than "krpyptonite".
SUPERMAN #8 - "The K-Metal From Krypton!":
Superman Krypto-Ray Gun ( toy ):
Kenner's Give-A-Show Projector: