Necking down .300 WSM to 7mm

[WARDOG]

Good day fellow shooters. I have been reloading for over 20 years but mostly for hunting applications and a few target applications. My most recent project is necking down .300 WSM brass to make 7mm WSM using Norma brass. I have never necked down, or "wildcatted" a cartridge so I have a few questions.
I am using a Redding Competition Die set: Seater Die, Body Die, and Neck die.
The.300 WSM case has a neck thickness of .0155"
According to my reamer dimensions, I would like to get a 7mm WSM neck thickness of .015". The reamer is for a standard WSM Match chamber, No tight neck etc.
Using the Redding Competition Bushing Die, I have bushings of .311, .312, .313.
Obviously necking down increases the neck wall thickness. Getting the O.D. necked down to .313 will leave the I.D. of the neck too small for the pilot of the neck turning tool, or to seat a bullet.
My dies do not have an expander ball to properly form the I.D. of the case. Do I need to buy a separate die with an expander ball to get that initial .2825 I.D.?
Is there a better way that I am not getting?

Also- I plan on fireforming 300 pieces of brass (Not sorted or culled yet). I have an unshot, Krieger barrel on a Surgeon action. I realize the life expectancy of the throat will be around 1200. I hate to use up 25% of the barrel life fireforming brass.
Has anyone else used a "Mule Rifle" (A inexpensive, or worn out rifle in 7mm WSM) to fireform brass to get it close enough to my chamber specs to start serious reloading?
I can't find a tired 7mm WSM chambered rifle yet so I am looking at purchasing a Savage or other inexpensive brand in that cartridge just to do the brass forming operations.
Any thoughts on that?
Thanks-
WARDOG

[mif_slim]

Can't answer you on the neck down part as I'm still trying to find ways to make my 300blk brass too, but on fireforming you can try the oatmeal way. Charge the case with x gr of powder, put a small cotton swab to keep the powder at the base and fill the brass with oatmeal then top it off with another cotton ball..point in a safe direction and shoot... Look at your brass is it's formed to spec if not add more powder until you fire form. You save the barrel life that way.