I"ve had my Marlin 1895 since 1977 and have reloaded for it the whole time.
500 gr. bullets are too long. You have to seat them deeper in the case to meet the 2.550" overall cartridge length. This can cause serious pressure problems. To reduce this danger, you must reduce the powder charge. The reduced powder charge may make it more difficult to stabilize the bullet. Consequently, you get lousy accuracy, a bruised shoulder, and the very real prospect of generating pressures that ruin your gun, face and day. Just not worth dinking around with.
Use the 500 gr. singly, in the chamber only? Might work. But the Marlin 1895 has a fairly short leade, meaning there's not much room for the bullet in the chamber, before it meets the rifling. The rifling is a constriction. Pushing the bullet (in the cartridge) against this constriction will do one of two things: push the bullet farther back into the case, or push the bullet into the rifling.
If the bullet is pushed farther back into the case, pressures will be raised. Soft lead bullets can be pushed into the rifling, but not jacketed or fairly hard bullets. The rifling will refuse to engrave into them. Consequently, you won't be able to fully seat the bullet and your rifle cannot fire.
Now, you decide to extract the cartridge and ... the bullet remains stuck in the rifling while the case slides off it and dumps powder all over the interior of the action. Now you've got a filthy action, and a bullet jammed in the barrel ahead of the chamber.
Which 450 gr. bullet are you thinking of using? Most are about the same length, and are probably are too long to feed reliably in the Marlin action. Remember, you're limited to an overall cartridge length of 2.550 inch.
In the game field, you won't gain a thing in killing power with that extra 50 grains of bullet weight. The .45-70 has killed game throughout the world with bullets weighing 405 grains and less. It's .45-caliber, heavy and if propelled at 1,600 to 1,700 fps will take everything but the Big 5. Hunters need to emphasize marksmanship over bullet weight, velocities and gadgets.
Were I you, I'd stick with 400 to 420 gr. bullets. I've used the Lyman 457193 flatnosed, 420 gr. bullet for decades. It's nicely accurate in the Marlin and will take any game in North America when pushed to 1,650 fps, as I load it.
