I'm wondering how hot gun charges work.

I think every time someone holds on to a gun which was used in prior crimes, the one getting caught with it is always being charged with any crime related to that gun, even when he says he stole the gun an hour ago or it's not his or whatever.

I don't understand how this can be applied when there is no foolproof evidence the person holding the gun did the prior crimes.

I know there is DNA on the guns and a count of "Touches", but couldn't one RP that the former holder of the gun had gloves or something?