Intriguing.
I've only fired more than a few centerfire rounds, but never once had to take a casting to check a chamber. Definitely have used go/no go gauges a few times. Back in the dark ages when I was competing in long range matches, the people cutting resizing dies always asked for a few pieces of fired brass and took measurements off of fired brass and never once asked for a casting. Hmmm......
Chamber cast are made to check the specifications of the chamber.
Sizing dies are cut using the fired brass, so the die can reduce the brass back tospecs so the brass can fit the chamber. It has to do with the the size of the brass not the chamber.
Fired brass is not the same size as the chamber, it is smaller.
Chambers have their specifications with tolerances. The custom die poeple aren't necessarilly looking for out-of-spec chambers, (they do find them how ever) They are looking to make a dies to reduce the brass to correct ammunition specs based on the tolerance found on the fired case.
Chambers are made to tolerences.
Ammuntion has tolerances also.
The die makers are making dies to address these variance cause by tolerances.
When a cartridge fires it expands to fill the chamber, then it contracts slightly. Brass is springy. It is smaller than the chamber but usually larger than specifications for ammunution.
Ammuntion specifications are smaller than chamber specifications. A cartridge wouldn't fit a chamber else wise.
Because brass is springy sizing dies are smaller than ammuntion spec.
Resize dies have specifications that reduce fired cases back to specifications in for ammuntion. The customs die makes cuts the reamer to meet those tolerances based on the dimensins found in the fired case.
A chamber is larger than a factory or fired cartridge.
A factory or fired cartridge case is smaller than a chamber and larger than a sizing die.
If a cartridge is fired in a chamber that is made with a reamer cutting to maximum specs-like with a brand new reamer, that case is going to be large. If a rifle has been shot for 40 years and especially if that cartridge is a high pressure cartridge, the chamber can be enlarged and wore, increasing the dimensions of the chamber. That change can only be found from a chamber cast-it can be indicated by enlarged brass.
If that case is then resized in a sizing die, a die reamed with a reamer with minmum specs that has cut a large number of dies, (factory dies) and has been resharpened a number of times it is going to be small. The die will still be in specs but it will be samll and tight.
Its called stacking of tolerances.
The way chambers are checked for spec is by chamber casting.
Die makers will work from both-castings and fired brass. My reamer makers prefer three fired cases or one chamber cast.
I still think this 40 year old rifle needs its chamber checked. Probably nothing wrong with it, and in fact all it need is a custom sizing die-made from three fired cases. But something is making that brass larger than normal and most likely is the chamber.
I got my knowledge from college, where I learned a 1.000 inch pin guage will not fit in a 1.000 inch hole guage. And in the shop when a customer supplied 500 rounds of range pick-up 30-30 cases to make into .219 Donaldson Wasp to go with the rifle I was building for him. First step full length resize all that brass in a 30-30 die. I also built a set of universal forming dies. 2500 steps later...
