A .338, or even a 458 will not KNOCK a Moose off his feet. When a shoulder shot is made the Moose will simply fall due to the sudden lack of support. Then if suffecient damage is done, he won't be able to get back up. With a smaller bullet the amount of shock from the bullet hitting bone, and muscle damage is somewhat less. In my experience usually not sufficient to cause the animal to fall, just to stumble. Allowing the animal to run away, and the pain in the affected area will cause it to keep running till it feels safe. To the middle of a big lake or pond.
I have examined several 30 cal hits on Moose, mostly 30-06. With a shoulder shot there is hydrostatic shock damage 6 to 8 inches, to the meat around the bullet hole, a hole through the shoulder blade, and a hole through the rib area with no hydrostatic damage to the muscle under the shoulder blade. Moderate damage to the top of the near lung, sometimes to both lungs. The bullet is usually found under the skin, or in the muscle on the off side. Again, no hydrostatic shock on the inside of the shoulder blade.
In comparrison the same shot made with a .338 Win Mag. Hydrostatic shock will be about 12 to 18 inches in diameter. A hole through the shoulder blade, more hydrostatic shock damage 6 to 8 inches in diameter to the rib area under the shoulder blade. Both lungs are turned to jelly. Then a hole out the far side with little damage, just a hole big enough to stick my thumb through. If a rib is hit the exit hole is then big enough to get three to four fingers through.
The 30-06 is my favorite cartridge, I favor it above all others, except for Moose and Big Bears.