Re: New brass - different velocities than fire formed?
Enough of that powder is lit & reacting to it's confinement, before ANYTHING moves or changes, that the affect in discussion barely relates to further downstream timing.
Yes the brass expands quickly, but much of the powder's burn rate has already been altered by it's tighter confinement. So unless something unusal downstream counters this, all will be influenced by it.
Given this, it makes absolutely no sense to rely on brass that isn't fully fireformed & stable in dimensions. Atleast, not for LR accuracy..
And you bring a good example, which is supporting of this position.
To heal this, many FL size brass -everytime.
This is rediculous in my view, as dimesions can be managed without all the side affects.
Fireforming(2-3 reloads) followed by minimal sizing is good enough.