For those who are interested in 450 Rigby reloading data. I have been playing with Hornady's 350gr FP bullet. Three loadings shot well in my CZ 550. All were using Horneber brass
1. 94.6gr of IMR 3031, and Federal 215 Mag primers. 5 shot average over my Master Chrony was 2553fps. I'm not the best shooter but I was able to shoot a 2" group with this load using a Caldwell Lead Sled. Still "jiggles the fillings though"

2. 95.6gr of H4895 with CCI 250 Mag primers. Again a 2" 5 shot group. Average fps was of 2509.

3. 107.6gr of IMR 4064, with CCI 250 Mag primers. 3 shot average fps was 3040. (2 shot error message on chronograph hence the 3 shot data). 1.5" grouping.

When I asked PH's in Africa for loading info for this round (as it's used by quite a few PH's and Park Rangers) they said that it's basically similar to the 460 Weatherby. They couldn't give me specific loads as they use Somchem powder (which is South African I believe). They suggested reducing the 460 Weatherby data by 6% as the 450 Rigby holds 6% less than the 460.
Haven't tried any larger bullets yet. Got some 600gr bullets a friend gave me. Not sure if I'm looking forward to shooting those