I have a 458 Lott that is damned stout. Never wanted to shoot a 577 trex.
Yep. I carried a .450 Rigby with 500gr at 2300 - 2400fps. It will get your attention, and clean your sinuses at the same time. Never saw the need to go any bigger and wouldn't want to regardless.
I've shot a .450 NE which to me was easier on the shoulder than the .450 Rigby. But double rifles weigh in at 11.5 lbs and that certainly helps.... a little.
The 450 Nitro was promoted at around 2150fps.
Geoff McDonald from Woodleigh Bullets told me once he chronographed most of the old Nitro rounds and they never lived up to full factory spec's and that market back then obviously didn't have the chronographs to refute factory claims.
Your 450 Rigby is the 416 Rigby case necked up (460 Weatherby without the belt) so will load up to 2700fps with 500grainers if you explore its potential.
Yes the theoretical vs the actual is eye-opening now that we have chronographs. The old doubles were happy at 2,000 fps. Not sure I would want to push them much beyond that. Modern doubles can handle the additional chamber pressure so push away.
I agree that the .450 Rigby/Dakota has potential over 2,400fps, but two concerns: 1. My shoulder 2. Most of the bullets available start to shed their jackets (softs) at 2,400 fps and driving monolithics at that speed only results in one dead and one or more injured if anything is behind what you're shooting at. So I was happy at 2,300-2,400 fps and shooting 500gr Woodleigh Weldcores. Perfect mushroom and can bust one or sometimes both shoulders on a buff. One shot and you're done.