I'm not too familiar with the strength of the various Martini actions, but the pressure with the .375 Fl. Mag. is much lower than most people think.

The CIP rates it at the same pressure as the .30-40 and thus below the .303 Brit. Using CIP tables and assuming the "piston area" can be taken from the O.D. of the case, the .375 gives only about 10% more thrust than the .303.

The Flanged version is a slightly longer cartridge leaving more powder room, and it was spec'd at a lower fps despite being spec'd with the longer barrels often found with doubles and singles. These add up to much lower pressure. The Flanged is rated at 41,000 as indicated by CIP's crushers, and the Belted is at 54,000. (CIP crusher ratings typ. run about 1000 above SAAMI CUP.)