realism has it's place. A player cannot know every demented twitch of a GM's mind. The only thing he can forecast is the rules laid out and reality.
Say a players character is going to jump out of a window to escape. The player realizes that his character might land just fine, take a banging up, but be able to limp away, or become a street pizza. He can either rely on rules or lacking them, on reality to base his descision on. It is unfair in my book for a GM to say 'oh, 2nd story huh? you fall and die! screw realism!' just like the flip side, jumping out of a 6 story window and the character is able to run away clean...screw realism is unfair there too.
I've had a system banging around in my head somewhat similar to the DnD rules where there are different 'levels' of weapons and armor of simple, martal, and exotic, for armor I believe it was just light, medium, heavy. Not everyone could gain access to martal weapons and fewer still to exotic, but everyone knew how to swing a club. Class types had access to certain ones, and could devote resources to opening up access to higher ones rather than learning a skill (the math professor can either spend his free time learning to shoot, fly a plane, appraise rare gold coins, etc, but not all of them) Each type was a prerequisite for the next. I though a similar system for firearms would be good. It represents the fact that just like real life people cannot look up everyhing in a rulebook, the characters don't get to read the game manual and pick from lists. What would a character really choose without a player guiding him. This is a rough system I worked out representing weapon access, and also weapon attitude, what less/more informed people likely to go for. Certain classes would have innate access to certain levels, but everyone could invest and gain access to upper levels.
basic level -cheap mouse guns like the 25 acp lorcin, as well as target .22s ala Ruger Mark III or bearcat or what have you. The idea here is everyone would have this basic level. Maybe no idea how to load it, but point gun, pull trigger. Also, 'a gun is a gun is a gun' Need a gun? get a gun! Either buy the cheapest one from the guy selling them on the street, or walk into a gun shop and say 'I need a gun, i've never had one before' No thought put into stopping power etc. All types have access to this
Light - (prereq basic)32 and 380 autos, snubbie revolvers (I restricted it to 38 long). Person knows a bit more, can load and unload it fine. Knows enough to want something bigger than a .22 or .25, but has other concerns beyond stopping power. Is going to use the gun more as an intimidation or an 'oh crap' fallback option, hence concealability is going to be more imporant than getting something powerful. Just need something 'good enough'. Probably will rarely shoot it, just carry it to threaten, or leave it hidden in the sock drawer. Person is probably going to have other 'tricks' to use before turning to firepower. Think the charming spy seducing to get access vs the CIA hitman.
Standard (prereq light)-4 inch 38 special, 15-17 shot 9mm ala beretta 92, glock 19 etc.
A basic working knowledge of firearms. The knowledge to pick a decent defensive weapon, select decent ammo for it, etc, but still a basic beginner in the world of firearms. Max 1 extra magazine.
Duty (prereq light) this was a catagory created to allow police/security/army types jump up to a 13 shot 40S&W type weapon, as issued by their department, for official duty and use, but 'off the clock' they don't care much about guns, if they have anything it is goig to be an old snubbie or maybe a 380. Note, standard isn't a prereq. Duty allows them to jump ahead to a good gun type, but if they want to go beyond that (say a cop that carries a glock on duty, but when he is sneaking around with the other characters he is packing his custom wilson 1911) they are going to have to invest in standard to continue to build up the chain. If you got access to heavy you would also access duty
heavy (prereq standard)-357 magnum revolvers, 45 acp pistols, plus speed reloaders or multiple magazines. More knowledge on the realities of ballistics drives these users to select more potent handguns, and also spend more time practicing, realizing that only hits count and as such not troubled much about limited magazine capacity. Honesty, I have always considered meshing this with standard. I always want a system where the players must think that both 15 shots of 9mm is good, but so is 7 shots of the more potent 45, but then it usually ends up that in realistic damagage scenarios, those extra rounds in the 9mms don't get shot anyways.
Exotic (prereq heavy) custom guns, performance center guns, 8 shot 357s, double stack 1911s, para-ordinance p14, raceguns, sig 210s etc etc. I also toss high capacity 45s from glock, H&K (and now springfeild xd) for balance as well. Generally guns that give some sort of quaility bonus (sig 210, performance center 627 V comp, etc) something a bit harder to find that a real expert is going to go out of his way to aquire. Note this is one of two options from heavy
extra heavy (prereq heavy) 44 magnum revolver, desert eagle in 44, and other major handguns. Note, if you would want a 44 magnum performance center revolver, you would need both extra heavy and exotic
super heavy (prereq extra heavy) 454 class revolvers, desert eagle in 50, other extreme handguns. I had this broken down before 500 SW showed up, i'd probably slot 500 and 460 as both super heavy AND exotic
I also built in a 'hinderance' called 'overgunned' were characters would continually take guns they couldn't handle, mainly for intimidation value (desert eagle 50) that gave the character serious negatives if he ever actually had to shoot that handcannon he was always waving around or bragging about
This type of system I think does a good job of preventing every player having every character, be it the computer hacker type to the biker-bar-bouncer from grabbing a desert eagle 50 or 454 raging bull, etc. The computer hacker might invest 1 skill so his the character packs a 380 PPK rather than a .22 Davis but probably not spend the 4 points to get a doublestack 1911. The ex-cop private eye or the bouncer would inherently have access to a higher starting level being built around a fighting class which might mean they are happy with a glock 19, or maybe they will spend 1 point to get access to heavy and choose a 1911, or maybe 3 to get a 454 raging bull, who knows. Either way, it is easier for the bouncer to pack serious heat than the computer hacker, each of them has the potential to get it however, if the are willing to allocate enough skill resources rather than learning other skills.