Welll, this is just my 2 cents; I'm not like...awesome or anything, but I guess I probably have more experience, overall with them, then most people here.
M9: Good all-round pistol. Medium RoF, medium damage, low recoil, easy to control, nice iron sights, medium capacity. Good when you don't have magnum ammo; if possible, always aim your targets head, unless they are moving too fast or you don't have time to. Probably is the worst pistol for defending yourself from somebody charging you for dogtags though.
M1911: Decent pistol, medium, but bizarre recoil, ugly, flat irons (some people like them though), second lowest capacity, medium-low RoF. Good for most 1v1 fights between somebody else with a pistol, but that's it really. It usually takes a whole magazine to get a kill, though you can sometimes get two kills with one. Like the M9, not to great for stopping melee attackers, but a little better. Good with/without magnum ammo.
Grach: Good for multiple enemies, relatively weak per round, nice irons, high RoF (for BC2 anyways), low recoil, high capacity. It's only decent for putting down knife users. The best trick to use the Grach, is to simply shoot somebody as fast as the game will allow you to, until you see them fall over. IMO, the best overall pistol in the game when you use Magnum Ammo. It's a little too weak to use without magnum ammo, but doable.
REX: Small capacity, strange irons, medium recoil, high power. This is handgun means business, but it sucks ass without magnum ammo. With magnum ammo, however, it will tear people apart. It's better to use this pistol if you like to take more time to aim; it only takes one headshot with MA at close range to drop somebody, so it's good for stopping people from knifing you if you stay calm.
M93R: High capacity, VERY high RoF, burst-fire, good irons, very low power, high recoil. Despite what people think, you ACTUALLY CAN use this pistol at a medium range, if you control yourself, but it isn't wise to do so unless they aren't looking at you, and if they are, they better have been previously injured badly. It's best suited for killing somebody going for dogtags, but that's about it. I carry it at all times if there is a team of SeCers against me. Just hipfire at close range; use your ENTIRE magazine on them (unless there is somebody else, but stiill, even then). That's about all it really exceeds at, is a CQC self-defense weapon, but it's really good at it. If you try to run around, and use it like the other pistols, you will die a lot, because it just can't handle anything past close-medium range.
That's about all I have to say for them as a whole; if you have any extra questions, just ask them individually.
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