The US Marines have long used sniper platforms in their helicopters, for which their snipers undergo training. The Maritime Special Purpose Force Urban Sniper course at Camp Hansen, Japan teaches (among other things) snipers how to set up firing platforms in Marine Corps helicopters. - Adam Geibel
The Israeli Air Force's Center for Weapons and Systems Maintenance is developing a special sniper's platform that will be fitted into Black Hawk helicopters. The system should be ready in about six months. Technicians used shock absorbers and a special suspension system for the four feet long platform (usually fitted with a Barrett .50 caliber rifle), which sounds suspiciously like the stabilized camera platforms used by news helicopters. The sniper sits in a seat attached to the platform, which can be swapped out between different helicopters. The platform is expected to greatly enhance the accuracy of snipers firing from hovering helicopters, which Israelis have rarely used as shooting platforms due to the vibrations.