Slightly off the beaten track (as usual)...

I am forming 9.3x57R cases from 9.3x72R parents. Everything goes along swimingly through the process until I seat a bullet. At this point, the dummy round will not chamber. I have traced this to the case neck being too thick.

My intent is to order a custom inside neck reamer but I am in a bit of quandary as to what diameter.

I will do a chamber cast (just as soon as the plumbing is done) to get the chamber's neck diameter.

So, two questions:
What clearance should I allow for the neck expansion in the chamber?
What interference in the neck (reamer diameter) should I assume for a cast bullet of a known diameter?

As always, thank you.

Minimum of .004" loaded round clearance. Minimum of .002" neck tension for single feed. Might want .004" tension when feeding from a mag.

Very helpful. The rifle is a single shot. Do you know if black powder would have any influence on any of these numbers? The rifle has not been nitro proofed (or 'proved' if you're British).

Don't have a clue about bp anything.
If I were you I would buy a neck turning tool instead of a reamer.
Id imagine your brass varies quiet a bit in thickness and controling that would be much easier by turning the outside over a mandrel than trying to ream cases.
I use a 21st century setup and it works well. Several brands out there to chose from.
The folks at 21st century are shooters and can help you get the setup you need it you have any questions. Of course id also be hoppy to help if i can.

If this were a bottle neck case, I would surely go that way. Being that it's a straight taper (think 38-55 as it's very close), I was thinking that turning the OD would leave a shoulder where it stopped and that might be a not-so-good thing? Of course, the reamer would move the shoulder to the inside; also maybe not-so-good?

I get the fact an OD turner would result in excellent and consistent neck thickness where a reamer only 'follows the hole'.

I'll have to pick up a 9.3mm spud and try it out. Thinking on it, maybe a feller could full length resize, then expand and then trim the OD where it "bulges" out from the expander? Hmmm......

Much to try out.

Anyway, thanks again and also for the offer to help.