I think one of the problems with a falling block action and the Martini in particular is it doesn't have much leverage in lowering the breach block, as pressures increase the primer has a tendency to flow into the firing pin hole which will lock up the block. I have had that problem with a 310 Cadet I have that has been re-barreled to a .218 Mashburn Bee, even though the block has been bushed. I had to use the hardest primer cup I could find to stop the problem.

Bob