The rear action screw protrudes into a channel in the bolt body, guiding its travel through the receiver. If the rear action screw is over tightened or not correctly inserted, it may cause the bolt to bind.
Scope mount screws are sometimes too long and if they protrude into the receiver, may cause the bolt to bind.
If you have recently removed the breech plug, and it has not been reinstalled completely, your bolt may not close completely.
