I had a Browning A-Bolt rebarreled to 6br Norma.
This was a case where I didn't design & hold the reamer. I let the gunsmith use his given that it's enough challenge just to rebarrel a Browning(extended tennon, epoxied, interference fit). This was J.Kolbe, and he did an incredible job on the chamber, fit & finish.
Anyway, Jim says it's a bit tight.
So I turn a culled case neck to an even .013" thickness, seat a bullet and try chambering(it failed to do so). I turn another to .0125" thick and find that it chambers ok.
So..
No-Go case = .269 NK(.013 thick, x2, +.243)[Chamber Neck]
Go case = .268 NK(.0125 thick)
For 1.5thou total clearance, I need to take necks to .0115" thick(.2665 loaded neck).
For 1thou of tension on .2665 necks, I need a .265 bushing, which accounts for .0005 of springback on my sizing. .2665-.265= .0015-.0005= .001 tension.
For reloading, I'll be using standard Wilson dies on this one.
I pickup a Redding titanium nitrided bushing in .265 and drop it in the neck die.
I use Lapua brass in 6br & I have a bunch. I picked out 200cases with low thickness variance(360deg). Of these the brass measured ~13.5thou thick, so it's an easy turn.
After fire-forming I lost only 7cases to H20 capacity differences.
That leaves me 193 diamonds. Plenty enough.
After a few firings, I sent several cases & a Redding body die to JLC Precision for minimal sizing/bumping.
This chamber & system has produced the lowest loaded round TIR that I have ever measured. It's low 10thous and prepolishing is required to see runout over surface noise.
This was a basic plan, far easier than required for a wildcat.
You can scratch it out on paper before ever beginning, but to do so you need all the brass your going to use for a barrel -in hand, so that you can measure it.
Similar plans are needed for a gun build from scratch, and this all begins with a bullet.