“we are speaking of semi auto rifles using bolt action cartridges.
why would you lower the damage”
Well, first of all, balance, but technically there is a real reason for that: in bolt action rifles all the energy from firing the round goes into propelling the bullet, and in semi auto rifles part of that energy goes into cycling the action of the gun. So, while not noticable in practice, the energy loss is there.
That was in fact the reason for that weird looking muzzle on both G41 rifles - german army was concerned about energy loss, so companies designing new rifles were told not to make holes in the barrel and not to make it movable. And since you can’t make a practical blowback rifle, both Mauser and Walther opted for the gas trap system, which uses the gas from the muzzle to operate an annular gas piston. This system turned out to nave no real benefits while being more complex, so the G43 rifle has a conventional short stroke gas piston system.