Advice needed ... bolt opens on firing!

[Son]

Have heard of a couple of cases previously that turned out to be the mainspring. The ends of the spring are in contact with the rear of the collar on the striker at the front, and with the inside of the rear surface of the bolt body at the back. When cocking the bolt, the cocking piece (and therefore the striker) is held at the rear by the sear and do not rotate. The bolt body is moved forward compressing the spring until the bolt is up against the rear of the barrel, then rotated to the right 60deg to lock.
If there are burrs on the ends of the coils in the mainspring, they can over time dig little groves in the surfaces they run on. With the spring compressed when closing a cocked bolt, the rear can rotated with the bolt body if it grips. If the front of the mainspring grips the rear surface of the striker collar, then the coil spring is torqued. It's not enough to stop the bolt handle from being locked down under spring weight against the locking lugs, but when fired, the bolt body can be opened by the spring uncoiling.

Hope the description is clear enough....
Here's a pic of the bits concerned

Attachment 45847


of course there is always the possibility that the locking lugs are worn/ damaged, or have been re shaped by some well meaning bubba