The side of the barrel is marked not to exceed 3200 HST. That converts to something in the neighborhood of 28K psi. Some guys have said they have shot them hotter than that and those "accidental" loads I took to Namibia where a good bit above that. Fortunately, I have still have my eyes and fingers. Yes, there is normally an engineered in margin of safety, but you can see how short and thin the chamber steel is and there is no transition from the the chamber to the thin wall barrels. Me personally....I choose to stay on the side of safety and within the engineered design limits.
I did try several different weights in cast with gas checks from 325-425 grains at .459 dia. They worked well enough but, I had several thousand of the Rem 405's on hand and just ended up using them. One year I used the 405's at 2000 fps in a #1 and a a Siamese Mauser 45-70 on some Gemsbok. They made HUGE holes you could literally push a golf ball through but none exited and the jackets separated. Called a Rem rep and he said the 405's were designed to work best at MV's under 1600 fps. So...that is the velocity I've kept them at and they worked very well.
One thing I discovered with mine, the harder I pushed it the less accurate it became. I think the thin/short chambers and thin barrel walls probably contribute to that. Pretty much any good 400gr hard cast or jacketed bullet at 1600 fps will be pretty effective on any NA game and African PG.
Have fun at the range.
I have several boxes of the 400 grain Speer SP-FN bullets that I can load up to Trapdoor pressure levels for the MP-221.
The Speer 400 grainers are much like the Green Box 405 grainers in that they are designed to perform best at moderate velocities.
The Remy 405 grainers are hard to come by where I am. I don't see Remy bulk bullets for sale on line or on the shelves in the shops and stores like they used to be.
