I take Taylor's opinion on the 400-360 with a lot of salt. He knocks it but praises other cartrdiges with rather similar ballistics. The same with other cartridges. I suspect that a good deal of this sort of thing is do to spotty performance on the part of the bullets of the day and has little or nothing to do with the caliber.

Purdey's states that back in the day it was their most popular chambering in its class.

Be aware that there are five different non-interchangeable variants on the 400-360 theme. Far and away, the most popular was the Purdey variant. Westley Richards had their own version which took a 314 grain 0.360 bullet and had a thicker rim.

Most rifles, regardless of make, are likely to be chambered for the Purdey version. A Westley Richards is likely to be chambered for the WR variant which takes a 0.360 bullet of 314 grains. The rim is thicker, which is no problem if we are cutting down 9.3x74R brass. The base diameter of the WR may be bigger, perhaps not enough so to matter. I have had Bertram WR brass that will not go into a Purdey die due to too big a base, but I don't put much stock in Bertram's dimensions anyway. All in all the Purdey version will be easier to put up. There probably isn't a dime's worth of difference in their ballistics.