JM says this is a "must have" tool to properly adjust your sizing die. You need one for every rifle caliber you reload.
L.E. Wilson has been
making precision reloading tools for over 80 years. The Case Length
Headspace Gage allows reloaders to measure case length from the base of
the cartridge to the shoulder, as well as the maximum case length. This
process shows the reloader if ammunition is being produced within SAAMI
This is a one-piece, non-adjustable cylinder-type gage for checking
fired and resized cartridge cases for cone-to-head and overall length
Used to compare cartridges against the minimum SAAMI chamber dimensions
Drop a fired case into the gage. Take a measurement on headspace.
This can be done with either a depth micrometer or caliper (Depending on
if the case is sitting high or low in the gage). If the case sticks
out past maximum headspace the firearm probably has a larger than
average chamber. The gage can still be used because it is being used to
determine how much resizing is required.
Now once the initial measurement is taken, slowly dial in the full
length sizer until there is a 0.001 to 0.002 drop in headspace. This
will give the measurement to resize the case just below the chambers
headspace in the firearm being reloaded for, giving a more accurate shot
and saving brass by not causing it to be overworked.
Once the brass has been sized, set the brass in the gage and set the
gage on a flat surface with the head side facing down. Now look across
the top of the gage, this give the maximum and minimum for trim length.