Wes350, the Kynoch website lists their loads for the various cartridges they load today and it seems they have stuck to the original specifications for these cartridges. They list the 404 Jeffery 400gr at 2125fps from a 28" barrel with 16 tons/inch2 pressure, the 416 Rigby 410gr at 2300fps from a 26" barrel with 18 tons/inch2 pressure and the 425WR 410gr at 2350fps from a 32" barrel with 18tons/inch2 pressure.

Of course these barrel lengths are not the normal length used on the old Mausers or any modern gun, most being 24" or less. The original WR rifles did have long 28" barrels according to Taylor which were unhandy to use in the bush.

If your Rigby velocity of 2350fps was indeed that and derived from a 28" barrel you would expect a drop of more than 50fps going to a 24" barrel. Even at 20fps per inch less barrel length which is low, your MV for the Rigby would be 2270fps.

As always there is only one way to find out exactly what the MV is of any gun with any load and barrel length and that is to chrony it. My Mauser 404 usually gives 100fps less than advertised factory loads so is around 2150 fps with factory stuff. Got the job done on some big Aussie buffalo without any drama and I suppose that is all you ask for.