Some trimmers base their measurement off the case head and some off the shoulder datum. Isn't the most important measurement, to get the best trim length, from the shoulder datum of the chamber to the barrel ogive? When the firing pin hits the primer it drives the case forward until the case shoulder stops at the shoulder portion of the chamber. Once the cartridge has stopped the important part is that the neck does not hit the rifling. Hence, the need for trimming.
I use an RCBS trimmer with the paddle handle and it works great, and it's fast. But it does occasionally cut undersize by .004-.005". There are a number of trimmers that use the case shoulder datum as the reference. Seems that this would be the best measurement to use for consistency.
I may have my thinking wrong here and if so please correct me.
Some trimmers base their measurement off the case head and some off the shoulder datum. Isn't the most important measurement, to get the best trim length, from the shoulder datum of the chamber to the barrel ogive? When the firing pin hits the primer it drives the case forward until the case shoulder stops at the shoulder portion of the chamber. Once the cartridge has stopped the important part is that the neck does not hit the rifling. Hence, the need for trimming.
I use an RCBS trimmer with the paddle handle and it works great, and it's fast. But it does occasionally cut undersize by .004-.005". There are a number of trimmers that use the case shoulder datum as the reference. Seems that this would be the best measurement to use for consistency.
I may have my thinking wrong here and if so please correct me.
** The cartridge neck will never engage the rifling.
** All cartridge dimensions are based on case head to end of neck (OAL) dimension, so far as I can tell.
** If one undersizes the case OAL by .004" - it's OK. Take a look at your micrometer blades set at .004" It's...nothing.