I think aiming the M2 is more a problem because mappers don't put the gun at a level that works. The gun has to be properly mounted to have it's sights at eye level when the player is crouched, or standing. Many of the maps I've tried to use the M2 in, the gun is just slightly too low, or slightly too high to aim properly.
Another feature I'd like to see with mounted guns like the M2 (in addition to using the grab key to select it), is that when moving the gun more than a slight bit to either side, the player model should move with it to keep lined up (relatively) with the sights of the gun.
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