...on my M2K4 that says something pretty energetic moves when the gun is fired.
Back when I first got the pistol, I mounted one of those Crosman lasers with the 11mm adapter it comes with to the rail on the '04. To my surprise, no matter how tight I made the clamping screw built into the mount, the laser and mount insisted on "walking" backward on the rail a few .001"s with every shot - just as if the recoilless SSP was a springer!
To stop it, I had to drill the mount for a second screw down lower and closer for better leverage and more force.
Something pretty violent happens in this pistol during the firing cycle, but I never figured out what. Could it be frame flex?