Take a shot and the breach opens ?

[Tom]

Hi
This just started happening today.
I should give a little history on this gun. I bought a DH action and a set of VH barrels 12 guage, put them together smoked them and have a nice tight on face gun. While I was at it I replaced the wear block on the barrels.
I shoot skeet with this and have shot over 1,000 rounds through it and it worked perfectly.
Today every shot the breach opened. I took out the bolt and it looks fine no damage. I lightly filed the wear block and made sure the bolt was making full contact. Haven't retried it yet but I don't think I improved anything, I really didn't see anything wrong.
Any thoughts ? I'm sure I'm not the first person this has had happen.

Thank you, Tom