There are methods and or techniques that would allow a reloader to determine the length of a chamber from the bolt face to the shoulder of the chamber without firing a case, I do not know of a reloader that has the skill, methods and techniques, but is it a safe bet there are a few that can.
I have no clue why you would choose the #10 Redding competition shell holder, I do not know why you would choose any Redding competition shell shoulder by # (number), I have a set for belted cases (#6), I paid $5.00 for the set of 5, anything I can do with the Redding Compitition shell holders I can do with a feeler gage, the advantage goes to the feeler gage, I have 10 options between .000 and +.010, Redding competition shell holders offer 5.
I would suggest you determine the length of the chamber first (before firing), fire formers fire first, if I was a fire former I would fire a case then measure the length of the case from the head of the case to the shoulder of the case with a tool that is erroneous called a head space gage, something like a Hornady/Sinclair head space gage, after measuring the length of the case from the head of the case to the shoulder/datum compare the length with the length of the case before it was fired from the head of the case to its shoulder/datum, if you did not measure the length of the case before firing, start over. the difference ion length between the unfired case and fired case measures the effect the chamber had on the case when fired.
Then there is that problem with not knowing the length of the chamber, the L.E. Wilson case gage is a datum based tool, when a case is inserted, or as ‘THEY’ say ‘dropped in’ the case protrusion indicates the length of the case from the datum to the head of the case, problem, it is up to the reloader to furnish the ‘rest of the tool’, most use their thumb nail, I use a straight edge, I lay a straight edge across the head of the case then measure the open gap between the straight edge and bottom of the case gage, again, if the case had been measured before firing the difference in the two readings would indicate the effect the chamber had on the case when fired.
I have a set-up table, using the set-up table is an option and is not as awaked as using the straight edge, I set the case up on the set-up table then place the Wilson case gage on the case. After setting the gage on the case I measure the gap between the table and bottom of the case gage using a feeler gage. Meaning the long case is holding the case gage off the table, all that is required if me is to determine ‘how much’.
Using the #10 shell holder is to assume the chamber is .010” longer than the minimum length factory store bought case from the bolt face to the shoulder of the chamber. In the perfect world the difference between the chamber length and minimum length cases is .005”. Remember, the standard shell holder deck height is .125” the deck height of the Redding Competition shell holders increases the deck height .002” for each number, as in #1 has a deck height of .127” etc., the #5 has a deck height of .135”, except my set, 3 of the shell holders are off by .001”.
Again, I convert go-gages to go-to infinity gages.
Correlation, not fair, there is a correlation between my chambers and my dies. my dies restore cases to minimum length/full length sized cases, the difference in length between my unfired cases and fired cases correlates to the adjustment I make when sizing a case as in adjusting my dies to and or off the shell holder, back to Redding Competition Shell holders, Redding Competition shell holders offer 5 options, with the perfect chamber that is go-gage length the reloader would use the standard shell holder to size a case that is .005” shorter than the go-gage length chamber. A reloader would use the #2 shell holder to size cases that are .001” shorter than than the perfect go-gage length chamber.
Confusing. Chambers are not perfect, chambers come in various length from the bolt face to the shoulder of the chamber, Redding Competition Shell holders can be used to size cases if the reloader knows the length of the chamber, again, I have an Eddystone M1917 with a long chamber, it is .002” longer than a field reject gage, not a problem, I form 280 Remington cases to fit my M1917 chamber, I shim the sizing die off the shell holder .014” when forming/sizing 280 Remington cases, my formed cases are .002” shorter than the chamber when measured from the usual places.
BUMP? No I rectified it as in altered, the 280 Remington shoulder was erased, it did not bump, it did not move, part of the shoulder became part of the neck and part of the case body became part of the case shoulder, when finished I had a new shoulder, the old shoulder did not move, I formed a new shoulder, I have no ideal when bump stops and forming/sizing begins, again, bump sounds like an accident.
F. Guffey