demo_450-
Here's a link to a thread from last September describing my experiences with 2400 and 185-grain bullets:
185-gr SWC, Chap. 6: A Brief Trial with 2400Summarizing, I put up to 47 grains of 2400 behind a 185-grain bullet, getting up to 2850 fps before I judged pressure to be high enough to make me stop tipping the powder bottle. (I'm working with a bolt action, so I don't have to worry about semi-auto operation.)
Because you're planning to use a bullet considerably heavier that I used, and because pressures are likely to build a lot faster with increases in 2400 than with 4227, please be cautious. IMR-4227 is on Hornady's loading sheet, but 2400 is not. We're straying into uncharted territory. Unlike 4227, with 2400 it may not be possible to use the same powder charge in both your 450 and 460. The 450 case has a smidge smaller capacity than the 460 and the 450 runs at lower pressures.
Did you happen to measure velocities with your 4227? There have been few reports of results of 4227 with any bullet weight, and it would be interesting to know what you found.
Is 4227 the slowest powder that might yield acceptable results in the 450?
--Bob