Jeez I can barely hit someone with a Browning sometimes why a sniper rifle?
Same thing that Seal84 said, vehicles moving plus units in them unfastened equals road rash or a fall to the ground.
These two objects don't work well together due to the game engine and there is nothing that can be done.
I'd prefer a M60 mounted on the bottom of the door of a Blackhawk for more range of fire. The browning ok except its got what 35 degrees of range. A sniper rifle would only work going slow but in a high fire area with dudes shooting and such, a slowing moving Blackhawk is easy prey for machine gunners, tank mgs, AA soldiers, and me with a RPG should it be multiplayer. ;D
Even the current Mh47 is messed up sometimes when in hold position, some guy shot the chopper with an AK74 trying to hit me and it wedged the chopper over enough to start me on a slide to the side and out the door. That russian got the pleasure of seeing my body fly 100 ft to the ground....
Guilty