amadeus76.... That is a common problem with the Mk.I*. The corners of the bolt release slot get worn or broken allowing the obvious forces on the bolt head to rotate out of the now larger span. It is quite repairable with a spot of mig weld and very careful dremel grinding. You will also want to use a small file and put a chamfer on the forward and rear edges on the slot in the bolt head to help prevent further damage.