Well, guns have been known as firearms for a long time, so it doesn’t take much imagination to switch the terms and call your arms guns. I’ve heard it mainly in a baseball context, where a player is said to have a gun (or a cannon) for an arm, reflecting the fact that he can throw a ball fast and straight like a bullet out of a gun.
Having arms described as “guns” was not unknown in northeastern Wisconsin, at least in the late 80’s to early 90’s. It wasn’t used all that frequently, however. I can recall several incidents of this, usually involving cheerleaders and games that required arm strength, like swimming or volleyball.
I can remember at least once incident where someone from the suburbs of Chicago (Hinsdale, IIRC) being very confused at the term. I never thought that it might not be a particularly widespread usage, as confusing someone from the suburbs of Chicago is not a particularly difficult feat.
It would be similar to the “gams = legs” slang, except that it’s more current. Usually when someone used “gams”, it was with an ironic, retro linguistic inflection, whereas guns was a more current term. However, with the current rate of social recycling, it’s possible that “guns” currently seems as outdated as “gams” did then.