Appears that once the case is EXTRA large for efficiency in that bore size, it takes a LOT more powder to gain even a little velocity.

We see this all the way from the .17 calibres, on up through the chain of command - in bore size.
I am referring to ammo loaded to the same modern specs. for breech pressure.

This is not to say velocities do not increase, because they do - or can. It all depends on how flexible the 'new' round is, how easy it is to develop accurate ammo, and how useful that extra "performance" happens to be - to the new owner.

It's fun to play in the Twilight Zone, but it can be expensive - but it can be fun.

When I went from .220 Swift to the .220 Swift IMP chamber, I gained 10% in capacity, and 0 fps with the 3 powders I was using. Maybe I was already "THERE" - or maybe I was past there?