Why does magnum movement matter for rifle accuracy?

[mustelid_master]

I've been thinking about this a lot and am hoping someone can answer it.

Magnum rifles are less accurate, partially because we shoot them less accurately, and partially because large magnum rifles move more during the shot. I would think that most rifle movement is caused by recoil, the vast majority of which happens after the bullet leaves the barrel. If the bullet has left the barrel, who cares how much rifle moves from an accuracy perspective, not a shot-spotting perspective. The movement caused by the change of center of mass from the firing pin coming forward would be similar or equivalent to that of a small centerfire rifle I imagine.

Where am I going wrong?

 

[Q_Sertorius]

In assuming that the majority of the recoil occurs after the bullet exits the muzzle.

Also, the firing pin moving forward does essentially nothing in changing the rifle’s center of gravity.