KWK

These old Martini actions were originally designed for the 577/450 Black Powder cartridge, and were later adopted for Nitro cartridges when the 215 grain Mk V1 .303 was in vogue. This developed about 17 tpsi, so giving the Gunmakers of the day their due, I guess they deemed the action satisfacory for that pressure AND cartdridge head size.

Original ballistics for the .375 Flanged Magnum developed slightly more pressure at 18 tpsi, but the base area is substantially bigger than the .303 (.456" for the .303 and .511" for the .375).

Actually, in calculating "Bolt Thrust" it is the internal sectional area at the base that factors in, and to do a proper comparision, you would have to section some cases and measure that, but by my estimation, the difference between the thrust acting back on the Breech Block would be in the order of 30% higher with the .375 Flanged Magnum; that is if it was loaded to original ballistics!

The .375 Flanged Magnum, being essentially a handloading proposition, there will always be those that will be tempted to drop in a heavier pill....a few extra grains of powder...and...?

Just sounding a note of caution.....