I do not use gages as in head space type, others rent and or purchase a head space gage. New, the cost of a reamer could be $140.00 + or -. The magnum uses a different bolt face and the bolt handle will need to be turned down for a scope and then there is the stock the barrel channel will require a lot of work unless the stock is replaced and then there is the feeding meaning a lot of work is required to get the large case to feed.
And then there are tools, lots of tools, some will make you wish you had never started the project like the barrel vise with the cute little blocks of wood, to buy time the manufacturer of the barrel vise will instruct you to find rosin for extra grip, I will tell you the little blocks of wood will burst into flames from heat before the barrel comes off of some Mausers and Eddystones (M1917).
The barrel will not be finished meaning it is not blued, I would suggest you start with an easier project as in going from 8mm57 to 30/06 or 270.
I am starting on another 2 builds, I make no claim to the bench 'rester' world, so this one will be more like a rifle in the crack shot world, My wife called the range and ask Dotty if she would give me a message, Dotty ask her if I was the good looking one, my wife answered with "No, he is the one that looks like he is handy" and Dotty said "We got one out here that looks like a plumber" and my wife said "That's him"
Anyhow, I have $25.00 each in the barrels plus shipping and $50 each in the rifles then there will be hours and hours of work with thousands of dollars worth of tools, and a friend said "Drop them off and I will do them for you".
F. Guffey