I have a M1917 rifle with a chamber that is .002” longer than a filed reject length gage. When sizing cases for that chamber I use 280 Remington cases. The 280 Remington case is longer than the 30/06 case from the shoulder of the case to the case head by .051”. I do not find it necessary to fire form, I form first then fire.
When sizing a case for the long chamber I adjust the die .014” off the shell holder then size the case, I could use a 30/06 forming/trim die first then size but I do not find it necessary. I do believe head space gages are nice but as long as they keep installing threads in my presses and on my dies I will continue to off set the length of the chamber with the length of the case.
F. Guffey