Chestsprings, I'm not an expert, but I'll give it a shot.
Imagine a bullet and gun that weighed the same. The gun would travel backwards at the same speed that the bullet traveled forwards (scary thought), each using 1/2 the energy of the powder.
Now imagine a gun bolted to 100 tons of concrete, which a gun in a vise essentially is. The bullet will do virtually all of the moving. The concrete would actually move a little, but scientific instruments probably couldn't measure such a tiny movement.
If the bullet in the 2cd example had a mv of 3000 fps, the bullet and gun in the 1st example would each have mv of 1500 fps.
A shoulder-mounted rifle falls between examples 1 & 2. The rifle does move back a little, and I have the scar from my first shot from a scoped rifle to prove it. <img src="/ubbthreads/images/graemlins/blush.gif" alt="" /> With some of the energy "wasted" in moving the rifle toward the shoulder, the bullet has to have a somewhat lower mv.
I never thought of this before, but it makes sense. Interesting post, thanks for bringing it up.
