Thread: Correct size neck bushing

Hey guys

I have got my hands on some redding dies for my 7saum but havent any bushings!
I see there are a number of methods for identifying which bushing size you need.
Using a loaded round and measuring seems to be out of the question as I am intending to neck down Norma 300saum brass so obviously cant seat round.
The second option of measuring case wall thickness, then adding bullet diameter seems like a possibility however wont the neck become slightly thicker once it has been sized down to 7mm from 7.62?
I saw the recommended bushings for the caliber range from .311 to .319 but at $20 a pop im not too keen on buying a heap and using a trial and error approach!
Sorry if I have missed something basic, have only used lee dies up until now
Any help much appreciated

Cheers