300nm was designed to be run through belt fed machine guns(as well as bolt guns), hence the reason for taper and low shoulder angle. Before tearing down the original design, a bit of understanding needs to known of its original intended purposes.Perfect explanation, I run 90% improved cartridges and hardly ever have to trim brass. You will get min brass growth running improved! As with any cartridge I load for, I do keep a close eye on the case head expansion at the .200 line and primer pocket depth. Pressure tested NO but a caliper and constant measuring will tell you when your into some excessive pressure.
300 NM has some of the worst brass flow or case growth I have ever seen. More case taper = less grip on the chamber walls. More bolt thrust perhaps.
I believe Jim Boatright has written a article on this matter.