Heard around the range."There's nothing like new brass". Why is that? "You need to anneal that brass". Why is that?
Is seating depth more about the distance to the lands or is it about the length of the neck holding the bullet? Is a coated bullet held less by the neck than an uncoated bullet? When does ignition pressure open the neck and wave goodbye to the bullet?
I don't declare I know the answers other than to say to look at the target.
Brass work hardens as it is manipulated.
Modern case production starts out with a moderately thick round brass slug punched out of a strip of brass.
By using a series of male and female punches with differing clearances the brass is stretched and formed.
It hardens up as it is stretched.
Annealing lowers the brass hardness making it less likely to fracture or tear and also requiring less force
to continue the work of shaping it.
To add to all that changes in the case during firing the chamber of the rifle also minutely changes size under the high pressure it is subject to
during firing.
But you are down into micro-inches of diameter change.
The change can be detected by things like strain gauges.
Capturing the changes in the dimensions of the gauge (it has a resistance element on it that changes value as the gauge minutely deforms) allows us to compute the stress that was applied to the metal since the strain (change in dimension) has been detected and captured.
You can actually put strain gauges on a barrel and detect when the bullet passes each gauge.
Luckily for use the steel is in an 'elastic' mode and returns to its same size.
It actually stretches but not far enough to be deformed permanently.
It then returns to its previous size what the stress is removed.
Bulging a barrel means the stress moved so high the steel was in a 'plastic' mode and deformed permanently.
The bulge stays.
.