A few things.

First, no, IMR and H 4350 are not totally interchangeable. You need to work up your loads with either powder using normal SOP and you should NOT merely yank a load out of a book, stoke some cases and shoot. Especially in that rifle.

As for the statement about not using loads with air space, that depends entirely on the powder. It is true that slow burners should not be reduced below manufacturer's recommended loads, but other powders of faster rates leave significant air space and are totally safe. For example, IMR 3031, AA5744 and H4895 all leave substantial airspace in normal .375 H&H loads that are safe to shoot.

Mike:

Get a few load books and read the introductory sections on loading. Get the basic equipment and start with your .30-06. Do NOT rely on forums for a basic education on reloading. You can get some wonderful advice and information on this forum, but now and then somebody may post something that is simply not correct, or even dangerous. In my opinion a fellow needs to be quite advanced in his loading knowledge before he reads too much on a forum where anybody can say anything, and at times they do! What sounds "right" coming from somebody who sounds like they may know what they are doing because they possess an air of authority may be complete hogwash...and dangerous to-boot. This is common on many forums. Take what is posted and research it. There are those nutcases who prime with C4, fill the case with dynamite and seat a slug of enriched uranium that they shoot out of a milkshake straw. But most bad advice doesn't come this clearly; sometimes the Devil isn't wearing a red suit with head full of horns and carrying a pitchfork. Sometimes he is wearing 4-inch heels, fishnet hose, a jet black pencil skirt and smells like wonder. Yeah, the devil is in the details. Be careful!!

In .375 H&H Mag {belted}, I have worked up several loads using 225 and 220 Hornady bullets and AA5744 powder that duplicate 9.3x57 performance {2400-2500 fps} and if you could get similar loads worked up to zero in your flanged version would do nicely for a lot of general hunting of pig and deer. For 5744 powder loads, call Western Powders/Accurate for suggestions. Ask for Johan Loubser.

Finally, "reduced" {lower than standard} velocity loads do not necessarily produce lower pressures. Fast powders driving bullets at moderate velocities may develop higher pressures than full power loads using slow burning powders and driving bullets at higher velocities. Also, before you have extensive experience handloading or are willing to accept some undesired consequences, do NOT merely substitute one powder for another even though it may be listed as the same or similar on a burn chart.