Ooops, I really goofed.

I went to Kynamco's site and got their numbers for performance of the early British rounds. I cranked these through QuickLoad to get a rough estimate of pressure. In place of cordite, I used Reloder 15 since modern reloaders report it regulates well. I get the .375 Flanged running about 30% more pressure than the early .303, which was a very low speed round in comparison, only 2050 fps. The CIP pressure limits for the .303 must be based on later loadings.

Together with the bigger base diameter, one then gets about a 2/3 increase in breech thrust over the .303 -- ouch!