At least some of these Marlins had really large barrel dimensions. As suggested best to slug your barrel first. I had someone who had trouble getting any normal sized cast pills to work in his Marlin 45LC. I suggested he try some 350 gr 458 cal cast pills designed for the 45/70. He could not believe the difference the larger bullet diameter made. That model I believe has a 16" twist so it can stabilize heavier bullets.
Before buying a heap of 458 pills I'd suggest seeing whether that size bullet will chamber.
