Re: seating depth question
Take this as a rule in reloading(just tryin to help here):
Pay no mind to any dimensions from any manual.
YOUR chamber provides YOUR dimensions, which YOU measure in reloading.
This consideration of what's going on is just what a reloader does.
You can of course seat 40gr bullets to the same OgvOAL that worked well from 50gr bullets. You may have to tweak the die a small bit to make up for any difference in ogive radius, and in results of seating with less force(for 40gr bullets).
I'm sure you realize that load changes are in order for a lighter bullet. And optimum seating in your gun can be different between these different bullets.