Thought I’d chime in.
The 1-96 twist seems awfully slow for a ~640 grain bullet. But hey, if they stabilize then it’s OK.
I have several double rifle .577 BP Express’ of various vintages , late 1860’s to late 1890’s. The ones that are known to have been intended for the heavy 650 grain bullet (1890’s) are in the vicinity of 1-32 to 1-36 or so twist, dependent on maker.
Those known to have been intended for 520 -560 grain bullets are about 1-52 to 1-56 twist
My one from ~1870 does have a 1-96 twist and it works best with a very “stubby” 450-480grain bullet.

Truly, one never knows until test firing. As well, it may make a difference whether the loads are black powder or modern Nitro for Black.

- Mike