RE: Leading a running Deer???
I agree in general that running shots are a bad idea. I've taken one in my life, and missed. With modern game regulations requiring you to basically have a DNA sample of the deer before you shoot it, on top of the difficulty of making a clean shot, I can't advocate running shots.
That said, I know some guys can pull off the running shot so long as the range isn't extreme. I think the key is doing a lot of bird hunting, which sort of gives you the instinct for leading. It's got to be instinctive, you can't rationalize it. A deer running just 20 mph is covering 30 feet/second. Even abullet at 3,000 feet/second takes about 1/10th of a second to cover 100 yards. In that time, the deer has covered 3 feet!
That's some tough shooting, and even though I was an expert rifleman in the USMC, I won't even try it. It ain't combat out there, and I want to KILL things, not maim them.