Those high speed camera shots have external forces holding the rifle back - however it is being held is reducing the amount of recoil. The more butt plate pressure applied, the less rifle movement will be seen.
Feel free to google “center of mass conservation”
Closed system means mass cannot be added or taken away.
Isolated system means there are no outside forces being applied to it.
Conservation of momentum says whatever the initial momentum is won’t change unless external forces are applied - if momentum is constant than so to is the center of mass. Bullet and powder go one way and rifle moves the amount needed to keep center of gravity in the same place.
This guy looks and sounds just like me - just much smarter. Lol