I shoulder this shotgun and shoot a deer with a slug. Strangely, the stock tapers to a point that's roughly 12 gauge.
The slug tears through the deers central cavity destroying the heart, lungs, and bones around it.. killing it quickly.
My shoulder doesn't even get bruised.
Fuck you, Newton. And fuck your laws.
The entire gun's mass has to be accelerated by the recoil, and since it is much larger than the bullet, it is accordingly accelerated much less.
That being said, I think it probably would bruise you. I've known people to get bruises from firing guns with entirely unmodified stocks, especially guns and relatively powerful cartridge configurations
a bullet kills both by penetrating and dumping kinetic energy into whatever it hits, force does not play a large part in the damage a bullet does. you are absorbing the force of the rifle's impulse into your shoulder when the gun fires, but not a lot of kinetic energy. It's essentially an elastic collision with initial energy stored as chemical PE in the propellant.
the reason for this is:
Bullet: Low mass, high velocity
KE=high because high vel
F=low because low mass
Rifle: high mass, low velocity
KE=low because low velocity
F= high because high mass relative to bullet
when the rifle fires, the PE is converted to a lot of KE in the bullet, and a little in the rifle, in opposite directions. because kinetic energy is what's going to generate a wound cavity, the bullet is effective in wounding. Equal force is generated but unequal KE is imparted to the bullet and rifle because of their different masses.
tl;dr study terminal ballistics
for clarification, I should probably give an example:
when you swing a golf club, your club doesn't go flying back out of your hands in the opposite direction of the ball. The club gives the ball an impulse, receives an equal and opposite force, but imparts a lot of kinetic energy into the ball.
every FORCE has an equal and opposite reaction. Kinetic energy is not a force and is not imparted in an equal and opposite fashion.
Where's the shoulder thingy? Holding a rifle or a shotgun properly should transfer the recoil impulse directly to your skeleton, creating no bruising. I can't think of any standard rifle with recoil so strong that it would create bruising when properly held. They probably just increase the contact surface area so it's better dispersed.