No one has said the sides of the case was going to split. What was mentioned before was someone was blowing primer pockets, which according to the real experts at Hodgdon, not we armchair experts, that when you have blown a primer, or expanded the primer pocket enough to loosen the primer, you have not only exceeded safe levels, you have gone far beyond them. Hodgdons data requires that the web area of ANY cartridge case not expand more than .0005", that's 1/2 of one thousandth of an inch.
Of course the case expands to fill the chamber, then the chamber contains the pressure, partially, the unknown here is the ability of the receiver itself, to withstand the pressure, on a constant repeated level of 375 Win loads. Like I said before, when Marlin says it in writing, I will believe it. Doing otherwise is at one's own risk.
I am fully aware that there are more than a few that have done this conversion, and no one has had it blow up on them, and hopefully never will. I would not load my Marlin 1895SS 45-70 to the Ruger #1 levels that are posted, just because it's the same chamber. Another example, is Marlins own design changes for the 450 Marlin, in the same 1895 receiver. It's said to be loaded to higher pressures than levergun loads for the 45-70, yet in order for them to produce it, they changed the barrel thread design to a stronger one than they use for the 45-70. Yes the chamber is one of the components that contains the pressure, once the case expands, the other parts, are the bolt assembly, the receiver where it supports the bolt, and the threads of the barrel. If the bolt holds up, the receiver holds up, the threads could still fail, little by little. Marlins own admission, is that the threads were made stronger to withstand the higher bolt thrust pressures. Granted, we are talking larger bolt faces, and with equal pressures, there will be greater bolt thrust from the 450/45-70 class cartridge. Ruger #1 load levels in the 45-70 reach 50,000 CUP, yet the levergun loads never exceed 40,000 CUP. In my mind, the 38-55/375 Win loads are like comparing the 450/45-70.
Again we are faced with the PSI/CUP faceoff, and there is no direct correlation to the two (SAAMI for 450 Marlin 43,500 PSI, 45-70 is 28,000. Hodgdons loads the Marlin levels to 40,000 CUP), yet Marlin did not feel comfortable with the standard design, the same one used on the 336, with the higher pressures of the 450 Marlin. So even if one wanted to TRY a direct number comparison of PSI to CUP, those numbers are very close, yet Marlin did not want the 43,500 PSI on the 1895 receiver, without some improvements.
Has anyone ever taken their factory Marlin 375 apart to see if there is any similar improvements not easily visible?
In the words of Jeff Goldblum in Jurassic Park, "You guys got so entangled with the idea that you could do it, that you never really thought about whether or not you should do it".
As for the thinner brass being just as strong as the thicker??? Since when is thinner brass, of the same type (ie cartridge brass, not soft vs hardened steel), stronger than thicker brass? We are talking basic metallurgy, and that just will not be the case. That's like saying 3/16" low carbon steel is just as strong as the same carbon level steel in 1/4". Sorry, not possible, not when comparing similar metals, which we are when comparing the rifle cartridges in question.
That thinner brass (38-55) will expand to chamber dia faster than the thicker 375 Win brass. And for the theory of case expansion, and chamber pressures, it get very complicated, depending on the powder used, brass, bullet weight, bullet design, and even primer used. Some powders reach full pressure much faster then others. A slow burning powder, could actually prove safer in this case than a faster one, since it will add a slight delay to the pressure peak. If the bullet has begun to move from the case, before powder pressure peaks, it will keep the pressure down. Yes slower twist rifling has been shown to keep pressure spikes down, a certain degree. Not sure if it's ever been noted just how much. Guys use the same philosophy with the 6.8 SPC to push it way beyond the factory velocity levels.
One last warning, then I am done. On straight wall cases like these, there are strong warnings given out by reloading suppliers, like Hodgdons. They say (told me personally, on the phone) one of the last things to show over pressure signs is the case head, because, being a straight wall (or close to it) case, the majority of the pressure is transmitted through bolt thrust.
Enjoy, shoot safe.
:-\
