Thanks for that, sthwestvictoria. Very interesting indeed. If there's more from the book then I would really appreciate it. I'm sure others would be interested also so perhaps posting them here would be good.
JDJ is saying the 375 is the best performer of his 444 based cartridges in a handgun but he speaks of jacketed's. Still, velocities for the 375 are higher than for the 357 which is what one would expect in a short barrel. But he is the shoulder for head-spacing while if I went 375 I'd be using the rim. I'd expect the 375-303 to at least equal the 375 JDJ because the barrel would be longer.
JDJ speaks of using the 444 for his 375 which in a way would be easier should one want a shoulder while the 303 is easy enough to neck up. However, the shoulder area needs to be ironed to the same thickness as the neck. This is done using a special die which I'm confident I can make.