RE: whats happening ( 22 hornet )
The nice thing about reloading is that you can try a lot of diffent things, for a reasonable price. If your brass is slightly under spec. after shooting, that may not be a big deal. Try shooting a few rounds with the brass under spec. It will not hurt anything, and you may have a slightly smaler chamber.
Since you have a single shot the only thing you need to worry about is the gap between the receiver and the barrel. If you have access to a "feeler gauge" then you should measure that gap.
I only neck size all my single shot cases, and I do not even concen myself with factory "spec". Chamber "spec" is all that really matters. And each chamber is slightly different.
Try the fired brass first. You might be pleasantly suprised. Tom.