Let's assume he had a 1350 fps load, 600 yards would be 55 foot or so holdover, not even trying to factor any wind drift.
I am assuming you calculated these results at sea level, because using the approximate atmospheric conditions around Salmon for that time of year gets 47-50', using a sight-height of 1" above the bore.
Another fact is both Keith and Kriley were shooting uphill, which decreases "drop" considerably. This is contrary to the common belief that shooting uphill increases "drop," even after decades (actually, more than a century) of various people attempting to correct it--including Keith, who more than once explained the need to hold a ghiher whether shooting uphill or down.
Also did some calculating with various degrees of front sight "hold up," and while Keith's story obviously isn't precise about how high he held the front sight, he does mention including some of the ramp. He also had the advantage of being able to see where the shots landed.
Dunno what the range was, but holding the front sight an inch higher with a 6-1/2" barreled .44 Magnum would result in about 30 feet of drop at 500 yards--without the uphill angle, which would decrease drop.
I might also mention the previous post about Brian Pearce reproducing the same shot at an actually lasered 600 yards with the same model of S&W and basically the same loads. I do not doubt this at all, due to having shot at steel gongs about a foot across with Brian (and a couple of other gun writers) at ranges out to 400 a few years ago. The range in question would only allow us to shoot either over a benchrest, or offhand (for some obscure reason), and over a rest (which would be much closer to Keith's shooting) we hit the 400 gongs more often than we missed.
From that we figured out how much front sight to hold over the rear sight, and started whacking away offhand. It was not entirely surprising (given the excellent trigger pull) that we usually came very close, or clanged them.
