M170 TM VSR-10 rifle troubleshooting.

Good mornin to yall!

Like I've told I own a TM VSR-10 rifle which I pumped up to M170 (550fps was it?). But it has been on the shelf for last few months because it has this mystical problem. The cylinder keeps stucking into the back position, if you try to move it with even the most little amount of force it will BANG! back to the front. I've asked my locals how to solve it: check sears, check nothing sticks in the way, check all parts are they intact... Sure, good tips, but they all are okay and yet gun DOES NOT WORK... Latest trick I was told to do is to check if the triggers metal plate in on the top of the trigger box is or is not bent a little. (If it would it would all so cause the backjam, but it seems to be okay too...)

???