I've been playing through Black Mesa and I learned that, except for me, no one has played any of the Half Life games. So I got to thinking, could I set a table top campaign in the Black Mesa incident? The problem is, I don't have a modern day setting ruleset at my disposal. I'm looking for something that can handle sci-fi, firearms, and some future-tech. Players would be characters at Black Mesa during the incident. I have seen d20 modern recommended a few times. Does the rule system hold up? Does it collapse under the pressure of over optimized builds like 3.5? Did first edition pathfinder ever come out with a modern rule set? Do I now have an excuse to buy Renegade Studio's GI Joe system?