dated slang, yes. But rather amazing nonetheless. In a culture that worships braggadocio we have a threat using understatement. I'll 'pop' not explode, boom or some other word denoting a huge sound and much force we have an understated metaphor describing a not so loud sound and alluding also to toy (pop) guns. A 'cap' not a bullet, slug, missile, bomb or other literal or hyperbolic synonym or metaphor but again a reference to a child's toy. Irony intended or not, it is rather wonderful and not quite what I would have expected. Then the finish. in your 'ass'. Not heart, head etc. Ass. Even though this threat is at least sometimes a death threat, or was, 'ass' is the term. A synechdoche, in a way. But since a bullet must enter only one part of the body, arguably one cannot simply shoot 'you'. But still the 'ass' is being used to represent the whole of the person and so it is a kind of synecdoche. Since one has a decent chance of surviving a shot to the ass the threat is again at least in some cases an understatement. It is also a shaming choice. I could shoot you anywhere, but I will since mark you and hurt you, pehaps with a hint of 'branding' as in cattle and property. I will thus, own you. OK. You think I stretch and you hate that phrase and the culture of violence around it. But perhaps if you set that aside you can appreciate the economy and richness of what at first seems something else.