Nothing wrong in terms of performance with the 450 Marlin. It’s popularity is muted by many who disdain its appearance (the belt), which has nothing to do with it firing a 400 gr. projectile at just shy of 2,000 fps. It holds about 5 grains less water than the 45-70, but is SAAMI loaded to higher pressure than the 45-70. Folks that push the 45-70 into hot at 42K-44K CUP where both Marlin cartridges operate put a great deal more stress on their bolts from Bolt Thrust by virtue of the 45-70s larger internal/external case head area. Eventually repeated hot loads at those pressures will degrade a Marlin or a Winchester lever gun. So you see the 45-70 reaching higher velocities but there is a cost to that performance on lever gun internals.
If you read most of Turnbull’s load developments and recommendations you’ll see he never exceeds 40K CUP and recommends staying about 38K CUP on 1886s with 45-70 and his 470 and 475 Turnbull loadings.
The 450 Marlin’s belt is less essential for worrying about chambering in bolt guns, etc., it is however pivotal regarding having a slightly tapered straight wall and needing something to headspace on. There is not a hill of beans difference in power and hunting game under field conditions between it and a 45-70. Unfortunately Hornady and Marlin both turned their backs on brass and rifle promotion. If you hand load the 450 Marlin you get versatility and a very potent lever gun same as the 458x2 American.
