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, an the .375 uses that size projectile, while the 38-55 uses between a .377 an .379.
Couple thousands of an inch might not sound like much, but thats all it takes to scatter bullets instead of droppin em in.
.38-55s have dimensions all over, some guns run as much as .380, others considerably tighter, brass also has been shortened recently. Marlin claimed their 38-55 to be right in the .379 range, but I know of at least one gun where the chamber was too small fer anything more than .377. Options here-chamber ream, or shoot jacketed.
Anywho, the reason folks tell all of you not to shoot .375s has to do with a few things-fist of all, the cartridge is shorter, an will not fill the chamber, if you follow up shootin with a standard oal number you could wind up imbedding the chamber, an at the very least creating a hassel of cleaning, at the most causing damage to yer gun.
Secondly, yer .375 is gonna be accuracy challenged at best, an at worst shootin that undersize of a bullet could wind up damaging both grooves an lands-at the very least, youll have a bunch of blowby, which greatly increases yer chances of extra work removin all that fouling.
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? I sure won't.
Last, if you need yer 38-55 to have greater velocity, then go to a slightly lighter bullet-its no sweat to shoot 220s or even 200s an get to or even better a little the 2,000 fps range.
If you really have a hankerin for the .375, I know there are still a few floatin around, save yer coin, get one of em, an keep that cb in shape.
Good luck n good shootin
