I have had my fair share of experience upgrading airsoft sniper rifles but have not come across this before.
I have a TM L96 and have just been replacing the 45 deg system with airsoft pros 90 deg trigger set. all seemed to be going well until i pulled the bolt back and its making a horrible metal on metal sound.
Upon investigation it appears that the cylinder is grinding at the sides of the spring guide stopper. Has any one had this issue before?
I am using a PDI precision cylinder, could it be that the channel is too narrow? When i run the stopper along the channel with no internals in it it doesnt catch. It is definitely here that the problem lies though as it has ground away dips in the side of the stopper.
The only other thing i can think of is that i took out the nylon rings from the receiver. there is minimal play there though.