In my opinion, allowing the usage of guns in RPs leads to gross overuse (and abuse) of them, especially if an RP is geared specifically to the 'post-apocalyptic/survival' theme (leading to the whole 'pseudo-rambo' thing you were talking about). There are some other minor problems that I've seen to some extent - the ease of use of guns sometimes leads people to put in less effort into descriptions and so on.

However, agree with you that total restriction might not be necessary. Firearms in post-apocalyptic RPs and in writing in general can be used as tools to create incredible, indelible drama, tension, and soforth. Like, imo, Cormac McCarthy's 'The Road' (post-apocalyptic) wouldn't be as moving if not for scenes with tense drama created in part by the use of firearms. The main thing is creating an environment where gun usage is more limited / changing your pool of RPers to one that you know won't go pseudo-Rambo on you.

Also omg Canticle for Leibowitz, what a cool novel.