Apples and Newton and shells and "foot pounds"
For each force there is an equal and opposite force.
When the apple fell onto Newton’s head his head received no damage, other than muss his hair.
The apple busted.
Another factor in the rifle recoil is the gas expelled from the muzzle after the bullet has left the barrel and is no longer subject to the expanding gas. This means the rifle is experiencing a reactive force which is not caused by accelerating the mass of the bullet.
This expelled gas also has force. The mass is small but the velocity is large. This is the principle by which rockets operate.
To demonstrate this effect you might pull the bullet from the case and plug the end or use a blank, and fire the rifle. There will still be substantial recoil. Perhaps not as much as with firing a bullet, but there will be recoil.
This does not take into consideration the total mass accelerated forward and the total mass accelerated back. But it does apply to the recoil force felt by the shooter and the impact force of the bullet.
So, stick a modified shell in that there clip and see how many of them there “foot pounds” you get.