Thanks very much for your help Daryl...

I measured the case neck thickness with a digital ball micrometer... and you are quite right there is MUCH more variation than I thought in wall thickness (I'm not a benchrester... and don't have a neck turner)

Measuring in three spots on each case gave an average of:
.01260"
.01280"
.01295"
.01260"
.01265"

So if I was to go down the PP path (I like the idea of being able to use fast soft lead HP bullets in the .450/400 for our local thin-skinned game) I should try to get the wrapped diameter to .410?

The Buffalo Arms 9lb onion skin paper is .002" thick, and according to their website two wraps equals a .006" increase in diameter (allowing for some compression after wet wrapping and drying)
http://www.buffaloarms.com/Detail.aspx?PROD=161977

So I need a flat or cup based mould throwing a .404" 400gr grooveless PP bullet?
Or slightly larger and size down to .404"?