Mak -
I don't know anything about the rifles that were chambered for the Newton, nor are they relevant, except its a reminder as to why I don't shoot antiques. The question at hand regards Marlins of modern manufacture.
By the way, 2500fps out of a .45-70 is perfectly reasonable - depending on the rifle used. In a Ruger #1 a 300g can easily be pushed that fast. Under 50,000 CUP according to Hodgdon. Definitely not something I would try in my Marlin.
On another forum I was blasted by the owner for suggesting that someone might find that .38-55 brass , resized in a .375 die, fit their Marlin .375 Win better than .375 brass, as it does mine and several other Marln 375's of which I am aware. He also blasted me for suggesting velocities that might be obtained by doing so, even though the velocities and pressures I was discussing were well below maximum for the .375 Win. The owner later admitted he had no idea what pressures or velocities were or what the situation was with Marlin 375 rifle chambers as he had no experience with the caliber. He was just blindly working off the idea that you don't fire a rifle using brass with a different headstamp than what is on the barrel. (Guess all the guys reforming brass into a different cartridge are doing something dangerous, regardless of the situation.) The owner looked like a fool and later deleted the entire thread.
The Marlin 1985 is chambered for the 28,000 CUP .45-70 cartridge, yet 40,000 CUP data abounds and Marlin doesn't complain. If the 40,000 CUP data was dangerous you can bet that Marlin would insist that the data providers not list the data as being safe in Marlin rifles.
It might be that Marlin, when Marlina was making 38-55 Cowboy rifles a few years back, went to the expense and effort of designing completely new receivers and made them substantially weaker than the receivers used in other 336's, when the raw recievers Marlin was already using would have worked just as well with no or only minor modifications to the final dimensions (like perhaps a slightly longer ejection port). The economics of modern manufacturing dictate reusing the same basic receiver, just as they have done before.
The situation is very much like that with my .257 Roberts, a Ruger M77. The .257 Roberts has a SAAMI limit of 45,000 CUP yet Ruger chambers the same receiver for cartridges that go as high as 54,000 CUP or 65,000 PSI. It is little wonder that I feel perfectly safe using +P data provided by powder manufacturers when loading for the Roberts.
There are a couple points here. The first is that the modern Marlin .38-55's are not necessarliy a grenade waiting to go off if the 30,000 CUP .38-55 SAAMI pressures are exceeded - it depends on the pressure being reached. Do I recommend doing so? No. The second is that each reloading situation needs to be evaluated on its own merits. Using +P data or loads is not necessarily unsafe simply because it is +P.
Is it possible to safely obtain .375 Win velocities in a Marln .38-55? I wouldn't try it for a number of reasons. Do I think firing a .375 Win in a .38-55 marlin is going to cause it to explode? Not likely, for a number of reasons, including the .378" bore diameter Marlin used in the .38-55s and resultant lower pressures. Could it? Its possible - stainless Sako rifles were blowing up with factory loads, too. Are the results likely to be satisfactory. Probably not.
