Stan,
I have been watching this thread with interest as I was given some 9.3x74R brass with a view to reloading it as 3” .410.
If you are concerned about pressure spikes in fire-forming the answer may be to find an old single shot .410, as long as it has a clean chamber, and use that for the fire-forming.
Most of those single shots have relatively massive chamber walls and the ideal would something like a .410 Baikal single which is almost indestructible.
The issue would be if the single-shot's chamber was oversize in comparison to the chambers Stan intends to shoot the ammo in. If smaller, no problem. This is based on my experience with .410s and brass hulls. Gil
