Slam firing means the gun shoots without pulling the trigger, usually as a mechanism malfunction with the trigger system or velocity of the firing pin returning to resting position, typically on semi automatic or automatic arms.
To have a slam firing sniper rifle would mean pulling the trigger would cause it to shoot continuously until you're out of ammo. Pretty sure that's illogical on a bolt action rifle
What do you really mean? Like mirrored? Left handed sniper rifle?