The BEST firearms system for D&D I've ever used was the firearms rules in the first edition of the old "Gamma World" game. The Gamma World system was close enough to 1st Ed AD&D that the rules translated over directly without the need for any additional tweaking.
The reason I liked them is that they were detailed enough to show the difference between the various types of weapons (handguns, rifles, shotguns, full auto), but were quick to resolve without slowing down the game.
If you could get a copy of those old Gamma World rules, you could probalby adapt them to D20 very easily. It sounds like the balance of level of detail vs. ease of play is about what you are looking for.
I like the Twilight 2000 system also, but wouldn't reccomend it for a "cinematic" game as the effects of gunshoots are too realistic for characters to have any chance to survive. It's also more complicated to resolve a firefight as well.
One quick trick you can use to show how different operator skill levels affect is to allow more experienced/trained characters or NPC's to get more bursts from a magazine than less experienced characters. Each burst has the same damage potentional, but the more experienced characters are able to control the gun better and use less ammo to get the same result. If, for example, a single 5.56 round does 1d10 dm, allow experienced characters to fire a three-round burst for 3d10 dm. Less skilled characters would still do 3d10 dm per burst, but would expend five rounds to get that result.
