The .375 was a stretch for the 336 action in the first place, as it operates considerably above the 336s safety threshold of 40,000 psi.
375 brass is heavier constructed than 38-55 in order to stand up to the extra pressure...
Third is that the .375 pressures are rated well over the threshold for the standard Marlin action, an this means you could run into head space issues real quick.
Sure, the ol .375/336s look identical, but will anyone swear that the newer cb recieved the same heat treating?...
Mak -
Marllin seems quite happy putting a 44,000 CUP (.444 Marlin) or 43,500 PSI (450 Marlin) pressure limit on the 336 actions, so 40,000 PSI is a little low.
Whether .375 brass has thicker walls is a matter of debate. I have WW brass in both and will check for myself one of these days, but others claim they ahve found no difference.
Firing a .375 Win cartridge with a .375" bullet is going to reduce pressures considerably when bore is .377" or larger. How much I will leave to others to test.
The receiver part numbers for my Marlin 375 and Marlin 336's of the same era are identical. What does that say about heat treatment? Also identical.
