You should just make a game in which sensitivity is realistically low in every scenario, pissing off the ultra hardcore twitch-shooter players, plus you'll have the excuse that it's realistic!
(disclaimer: all of the following is written from the perspective of Halo's multiplayer)
People always rag on auto-aim, but I actually see it as a positive influence in the FPS genre: my aiming skils are pretty average, however because of the mediating influence of auto-aim, the players less accurate than me shoot almost as well, and the players more accurate than me shoot comparatively not as good as they should: it acts as a handicap that evens the game out, keeps thing challenging for those who would otherwise get bored of mowing down noobs and, most of all, puts more gameplay emphasis on tactics rather than just aiming ability:
With auto-aim, you can be more sure that any given player will have at least SOME ability to shoot you, so you know that if you start shooting at the same time, the likelihood is that you will either both die or you will only just escape and be killed with one shot by the next guy to spot you. This allows players to succeed not only by being able to put a cursor over a hitbox and press a button, but rewards INTELLIGENCE. In a game like Halo, at least, encounters become more about who can, on the fly, make the best use of the environment, who spotted who first and who gave themselves the best predetermined advantage in terms of grenades, weapons and equipment they have on their person.
Basically: in my opinion, auto-aim favours those who are less skilled but more tactical over those who are more skilled but just march into a situation guns blazing because they are confident nobody can out-headshot them.