I'm soon to open my "Paintball Gym" in Gillingham, So when it's set-up Mtbryant more than welcome to come along and use it, as too any members of CPC or new recruits. 5 Max bookings an evening times to be annouced (6-11PM / Mon,Wed, Thur)!!!
The trick with snap shots is to try and make as small a move as possible. Try and get the distance from your in cover and shooting postions as short as possible. This way the action of popping and shooting will be as little strain on the body as possible, aiding consistency and allowing you to do it for longer.
If you haven't already, make sure your marker is correctly setup for your body size. Nice little video here: