Ken-
Your response is really helpful.
Your rifle's 8-3/4 lb weight will be appreciated if you continue to work with 450B loads at the velocities enabled by the fixed-breech construction. The 200-grain load at about 2850 fps is approaching recoil generated by the 300 Weatherby.
(For comparison, the barrel on my heavier bench rifle is 26" long, 1.200" dia at the breech, tapering to 0.840" at the muzzle. No fluting. Other description is here:
A Second Bolt Rifle.)
The c.o.a.l. of your load is a tad longer than mine (2.180"). I stuff in a bit more Lil'Gun, and find a heavy side crimp into a groove is needed for good ignition.
Thanks, and as you develop loads I hope you post results in the 450 handloading segment of the forum.
--Bob