Thanks all, some great options there... especially a lot of obscure stuff I wasn't familiar with.
I will be running a "Men in Black" campaign setting based off the movies -- with all the corniness included -- so I guess the setting can be adapted for a little less than "realistic" set of firearms rules
I have been running a post-apocalyptic setting where the party (a group of soldiers) was thrown forward in time 150 years, only to find that their world had been nearly decimated by a world war. (Oddly enough a lot of the things that I scripted over 2.5 years ago that led to the world war are actually happening in the real world... if China reclaims dominance over Taiwan, reverting its government to a communist state any time soon, well lets just say I'm building a bomb shelter
I do like d20's character customibility, although I do prefer a point based build system like GURPS. I'm surprised that D&D hasn't gone to something similar -- after all it seems silly that they keep adding new base classes and prestige classes to the system just for the sake of giving the character a few different "talents". This is where I think Grim Tales was heading in the right direction by just saying that at every 2nd level you get a pick of a talent. I'd rather see talent selection limited by pre-requisites, than by a class type through -- this way you could just eliminate the class system. It wouldn't be a point buy system, but would be fully customizable -- and that would be just as good.
In my post-apoc campaign, I executed a total party kill by running forward 10 lvl 4 barbarian mutants. The PC's were all lvl 8, and were built from RPO's Blood & Guts soldiers options (I threw in a few bonus feats to boot, not to mention a generous 32 point attribute point buy). Quite frankly, the PC's were designed to kick butt! The mutants took a round of gun fire, but with a full round of running and a charge attack, (closing from over 200') they laid waste to the party due to their high strength bonuses. (In a properly designed firearms ruleset, the mutants would have been riddled to death with bullets from automatic rifle fire by about the 100' mark).
That was the second adventure, and no one had any interest in playing the d20 modern ruleset after that. One of the players is an active soldier and he was really shaking his head.
I was considering chopping up the traditional 6 second combat round into further segments to account for firearm discharge -- but I was concerned how it would mesh with the other combat system mechanics so I instead decided to add in a few of my own house rules to enhance gun play -- which the players have been fairly satisfied with so far, despite it also meaning that they get shot up pretty good too
In summary, I don't mind the d20 system, but I find it lacking in combat mechanics when melee and firearms combat are intermixed. A few house rules can fix it, but I'd rather spend more time creating setting/adventure info than tailoring house rules.
One of the elements I liked from Classic Deadlands was how they handled wounds -- specifically hit location and the ability to make called shots. Such a system would probably be too deadly in a modern scenario though.
On a side note: RPO's Blood & Guts supplements are a great option for a military campaign!
Anyways, thanks to listening to my general ramblings, and thanks a lot for all the suggestions, looks like a trip to my local game store is in order.