Like I said, I don't understand why.
If you do it simply, it'll just be a weird version of your average FPS, minus the actual killing.
If you do it 'properly' and make it as realistic to airsoft as possible in a game, you're severely reduce the audience. To about 3 very hardcore airsofters. And I hate to tell you this, but for the cost of making the average game (heck, even simple indie games cost upwards of £50K to make), 3 people spending £40-60 on a game isn't going to be successful.
BatHat, your idea sounds oddly similar to two things currently on the market.
They're called Rainbow Six:Vegas Two and Call of Duty: Black Ops
Marksman556, I know why. You could do spring changes in game for the same reason as people in real airsoft. To cheat in game to have a far higher FPS.