Normal SAAMI headspace on a rimmless ctg. is .006 -.007" US and Euro, but normal belted cases do not follow that criteria, it's common for a custom 'tighter' chamber to fail to chamber some belted ctgs. This is the main reason, I distain belted magnum brass.

You'll find measurements on belted brass running base to top of the belt from about .210" to .224"(at the most) in some brass, ie: .014", over double what it should be. (except for .450 Marlin alone, which are longer yet from the base to top of the belt)

When I chamber a 'rimmed' or 'rimless' ctg. in a barrel for myself, I ream it at '0' headspace. You cannot do that with belted cases as only a few from a box or 20 would fit. The only recourse, and a good one if this is important to you, is to measure all in a box and segregate them. Chamber the gun for 0 to +.001"(the shortest to .001" longer), ie: .210 to .211", then turn the belts of all the rest to that length - a simple task with a small lathe. Then, and only then, will headspace be perfect on a belted chamber.

Lets say a factory chamber allowed any belted case to chamber, as most do. That means it has at least up to .014" headspace on some brass. Everything shorter than that, then has up to .014" excess headspace. Upon firing, the case stretches at the web, which is the start of separation and the case will continue to stretch there, every shot. The only exception to this is a friend's factory Styre Mann., in .300 WTBY. It would chamber only 1/2 of a brand new box of factroy ammo, due to the belts in them being too high.

Using my example of fitting the brass to the chamber, and fitting as per necking to a crush fit before fireforming, the brass will not stretch at the web when firing. The brass of that .416 Taylor will then have the potential to never stretch and last, virtually the life of the barrel. This is provided that the reloader never pushes the shoulder back and never exceeds normal maximum loads for that ctg., which run about 63,500PSI. When firing, if the brass elasticity is never exceeded, the case will shrink .001" everywhere, as it should. If the cases have to be 'bumped' at the shoulder at any time, your load is too actually too hot and should be reduced. I've brass that has been fired with what many people consider too-hot loads, yet I've shot them over 40 times each with those same loads, and only every neck sized, never touching the shoulder.

If this is 'old hat' for you, I apologise.

Perhaps the best chambering tool for making a .416 Taylor, would be to use a .300Win. mag. or .375 WTBT Mag. reamer body, with a bore sized pilot, then a neck/throat reamer to finish the job. That chamber would have no belt and would actually be a semi-rimmed case(.532" rim), headspacing on the shoulder, smae basic case shape as a .220 Swift or .356 Win.). Turning the rims to .500" would still work in a magnum bolted Mauser action perfectly, and you'd have a rimless .416 Taylor - wonderful round that would be.