The actions for both Marlins are the same. Some of the Marlins used for higher pressure, ie. 375, 444 have different barrel threads where the barrel ties into the receiver. Some are square and some are v-shaped. One is more for higher pressure supposedly than another. Don't ask me which one is the higher pressure, because I can't remember. The 450 is rated to 43,500PSI and the 45-70 to 40,000CUP loads which are about the same thing. One is the older measurement, of course. Loads for the 45-70 are confusing indeed. Some trapdoor loads are limited to 28000 cup, ( which is SAMMI), and which Marlin recommends. Some are rated to 35,000, which I believe Speer does, some like Hodgden will include 40000 and 50000. The 50000 is a load meant only for the toughest 45-70's, namely the Ruger #1 #3, and Siamese Mausers. You will get more velocity from a Ruger #1 than from a 450 because of the pressures involved and the limits of the gun. The two Marlins (45-70 and 450) are virtual ballistic equivalents. I don't believe the barrel threads do too much myself. Most plinking I do is with pressures in the 18000 to 30000 range. My hunting load closes in on 40000. I own a Ruger #1, and see no reason to load it too its full potential. Recoil in that little rifle is severe, and I am satisfied with 40000 cup loads.
