You can do load testing while the barrel is still speeding up. You just cant finalize it, generally. A lot of great info can be gathered in the first 50 shots rather than just waste.Definitely planning on keeping a log with this barrel. It's not something I've done before, but want to start.
As to load development while running the break in and checking velocity, how will I know if the velocity change is due to powder increase or just the barrel speeding up? I would surely like to do all simultaneously, but don't fully understand how it works