Neal Bloom, J Newell, Victor Lewis (and others) . . .
Permit me to attempt a different approach to Neal's question: "Can someone explain to me why a 125 grain round is harder on a gun than a 158 grain round? Is this specific to Smiths or is it true of all manufacturers?"
If we go back to basic physics -- Newton's Laws -- we shall remember that for every action there is an equal and opposite reaction. Thus, forward energy vectors must create equal, rearward loads to be absorbed.
Since:
1) Energy varies with the square of the speed (E=MVV/2)
2) Lighter projectiles/bullets will have greater initial velocity (given the same propellant) and much greater muzzle energy, due to the exponential nature of E=MVV/2
Therefore, the firearm -- and the person holding it, for that matter -- will necessarily have to absorb more "reactive energy" from a lighter, faster projectile than from a heavier, slower bullet.