Actually, I have several questions regarding various details surrounding this entire scene as it plays out, so it might just be easier to walk people here through what I had in mind, with "is this plausible? If not, what can I do instead?" questions at every single step.
1) Character A has a revolver. Its caliber is never specified, but probably somewhere in the .22 to .38 range if I had to guess. You know, gets-the-job-done but small. Not a hand cannon, basically. Anyway, he decides to sabotage it by sealing the barrel, with either a glass marble or some sort of steel ball bearing. (I want it to be the former, but can make it the latter if the former is just not possible.) He probably does this by opening the cylinder and inserting the marble/ball/whatever from that side. I want the size to be such that it's possible to do this, but enough of a snug fit with enough force required to make it permanent once done. He has access to basic entry-level welding/soldering-type tools at this point if that is necessary to get that thing in there forever.
2) He carries the disabled revolver around in a hip holster until he gets in a fight with Character B, who steals it. He accomplishes this by jumping onto Character A's shoulders and attacking his head (A is physically huge and B is emaciated, so the size and strength to pull this move off should not be an issue.) A can easily lift B up and throw him off, but B pickpocketed the holster with his toes while they were attached, so he gets away with the gun after being thrown.
3) Character B attempts to shoot Character A with the disabled gun (assume that he is ... not in the right mental state to do a thorough check of the barrel and everything, so he can miss this detail.) This brings up my biggest logistical questions; what I want to happen is enough of a miniature explosion to burn his hand and make him scream in pain and drop the gun, but not enough to blow his hand completely to bits or anything (there's still more fighting to do after this, so he still needs that hand.)
4) I would also like the marble/ball/whatever to detach from the gun in a way that isn't being fired out; maybe the whole gun blows up and it just falls out the newly-made hole, or maybe even get propelled out the barrel but only by a few feet or so. Basically, I want it so that someone else investigating the scene of this fight later, when looking around where the gun was, can find the marble/ball/whatever. (This is the main reason I realized I might need something more durable than a glass marble, if I want it to be recognizable and retrievable later. Damaged, sure, but not disintegrated.)
As always, thank you so much for any input you can provide! This community is so cool.