Actually, his first experiments in hotrodding a big bore cartridge was the .45 Long Colt, but unfortunately he made the mistake of attempting to load a .45 Long Colt cartridge with a .45-70 bullet, and the combination of that plus the thin cylinder walls of the Colt Single Action Army he used in his testing resulted in a catastrophic failure, so he gave up on the .45LC in favor of the .44 Special, since the SAAs in .44 Special had thicker cylinder walls.

Many argue that if Keith had only been wiser and hadn't attempted to load a .45-70 bullet (which has a diameter of .458 as opposed to the .452-454 diameter bullet of a .45LC) then such a catastrophic failure wouldn't have occurred and Keith may have very well gone onto make a ".45 Magnum" but that's besides the point.

Keith was actually a bigger fan of .45LC than he was of .44 Special, but switched due to the aforementioned event.

In hind sight, things turned out for the best regardless, as Keith was satisfied with .44 Magnum, and the ".45 Magnum" as it were came into being regardless in the form of the .454 Casull and later the .460 S&W Magnum.