D'Arcy is an impressive guy and is very generous with his time and expertise. I sure enjoyed my conversation with him.
Function is king with DG guns, no doubt. With the .300 RUM M-70 donor I'm using for my .404 J. project, the mag box, reportedly, is a bit wider than the std. mag. box and, of course, is full mag. length. So, with the rails and follower all set up for the fatter RUM case, a good smith should be able to make the parent round, the .404 J., work. The RUM's have a .534" rim, so the full width (.545") .404 J. rim will require opening the bolt face.
With the std. mag. action, .534" bolt face and std. mag box, a .423" bullet from a .375 H&H case makes a lot of sense. A .404-375 should perform right with the .404 J., unless one was trying to run up the pressure and make a Wby out of it. Then, the bigger case would outperform the smaller one. With 400 gr. bullets at 2,400 fps, there should be no difference.
DF
