I was using foam filler just like I use in my doubles.
89.25 grains of RL 15 would be the standard "foumula" load for 75 grains of Cordite. That was the load marked on the bbl flats.
Nothing out of the ordinary had occured in any of the previous range sessions.
When bbls let go in the "middle" it is not due to high pressure. The pressure is greatest in the chamber area.
This rifle suffered the classic bbl split/burst loosing about a 6" piece of steel. In the other bbls I have seen, they split to the muzzle, with one splitting back toward the action as well.