Also note that Al mentions high frequency vibration in addition to the recoil and forced deformation.
I don't think he states it in that page, but there would be pre-ignition vibration from the firing mechanism, then primer ignition, powder ignition, bullet slamming into barrel, etc. These vibrations would propagate thru the rifle at the speed of sound thru rifle material (steel)... in other words... really fast. But it seems that the recoil and deformations are larger than those smaller vibrations until the bullet leaves the muzzle.
