The modified case holds 93-94 grains of water. Now my reference material lists the 375 Flanged Magnum at 97+ grains of water and the 369 Purdey at 102+ grains of water.

I'd expect performance with the pressure limitations to be about 200 FPS slower than the 375 Mag or 369 Purdey.

Bill

P.S.- I also don't know what to call it if i make it. 400/375 is already a rimless round. Perhaps 375 Flanged #2?