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

I've got the same dies and brass. Getting a 7SAUM done at the moment so will check what bushing I have tonight and take some measurements.

Ok Had a measure up

Norma 300SAUM brass neck wall thickness 0.0155"
Norma 300SAUM brass neck sized to 7SAUM 0.0160"
Norma 300SAUM brass neck sized to 7 SAUM with 180 berger 0.3155"

Redding Bush size used was .314

Projectile seated using Forster Benchrest seating die

Legend cheers mate!

Yep i'm running a .314 bushing as well, with both Remington 7SAUM and Norma 300 necked down.