Hello Don.
Sorry that I hadn't noticed this post sooner.

I don't believe that you have a headspace problem.
You have a neck expansion problem.

Take a fired case and measure the dimension of the neck.
Compare this to the same dimension (with the same brass) on a loaded case.
You should have at least a few thousanths difference between the two, the loaded case being of smaller dimensions that the fired.

If the brass has no place to go when firing, you get instant overpressure.

I have a Dutch 6.5x53r with a .268 groove. The largest bullet that can safely be loaded into the cartridge is a .264 because a larger bullet would give no room for expansion.
Apparently, the long bullets were supposed to "set up" when fired and the accuracy was fairly good.

Check those numbers before you go any further. Problem could be solved.

Good luck.

Tom