Capt. Curl,
Not a fluke at all, Purdeys and H&H's like em too. The .450 BPE is by far the easiest express to load for. Don't even try with PP and black unless you really want too. The thin jacketed bullets are real easy on rifling, not like that erosive paper, and the pressure generated by the 4198 load are considerably less than that generated by black. See Grahem Wright's "Shooting the British Double Rifle" for pressures of many loads.
It is hard to get enough velocity with modern black for the barrels to regulate. Nitro for black is the way to go!
Best,
Mal
