As gun initially
accelerates rearward, it compresses the tissues of the shooter's shoulder
as it also compresses any recoil pad material; hence, as the gun moves
rearward the mass of the shooter's shoulder is progressively more
solidly tied to the mass of the gun. The further back the gun moves
during this initial acceleration phase, progressively, the further
back the shooter's shoulder must move and the greater the effective
mass in rearward acceleration. Explicitly, the further the gun recoils,
the harder it is for the accelerating bullet and propellant
to move
the gun back further!
Similarly, during this process the gun progressively becomes more intimately tied to the shooter. Moreover, the tighter the gun is bonded to the shooter the more the shooter can prevent the gun from rotating. Since the center of mass of the gun is not aligned with the bore axis, the gun will inevitably rotate during recoil. The butt will move downward and the muzzle will move upward. The tighter the butt is bonded to the shooter's shoulder, the less such downward movement will occur. Hence, it is during the initial acceleration of the gun that the shooter is least able to control any non-axial acceleration – inevitably, if the gun initially accelerates faster, the sight picture will be more disturbed. Anyone who has compared sight picture disturbance with various bullet weights in the same gun is familiar with this – lighter bullets allow one to see impacts at closer ranges, despite those bullets having less time of flight toward such targets.