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.