2) Progressive rifiling (aka "gain twist") can cause more deformation of the bullet which can lead to less stable flight.
With the gain change, the angle of the rifiling changes, so rather than 1 set of grooves all at the same angle, the rifling marks on the bullet "wipe" a larger area on the jacket.School me. I'm not sure I understand why you'd think this. I would think that a gain twist would deform the bullet less. My reasoning is that as soon as it hits the lands, it would be accelerated into a twist more slowly than conventional rifling which would cause lower forces initially on the bullet.
We are talking rifles:
When the bullet exits the case it goes from, zero to over 2000 fps in a near instant. The optimal barrel would be a smooth bore, but the down side is stability, so lands and groves were added to provide that and works quite well.
But in the windmills of my mind I think if we start the lands and groves later into the process we can optimize the velocity. The progressive rifling provides a more gentle twist approach this 'scrubing off less velocity while provides the needed stability.
polygonal rifiling: Seems to be going with a deeper throat to optimize the velocity and combine that with possibly polygonal - 5R could provide some advantages in both increased velocity and the solid stability required for accuracy.
MR seems to have a solid fan base in the Marlin .22 guns and those that have seem to think it provides better accuracy but has a downside on cleaning since there are so many groves.Thoughts on micro rifling?
and there are more, the Ballard and the Micro and ++ is my guess.