LR-

The rate of twist in the WR is 1/42"
The Purdey has a similar rate of twist.

You could do well with roundball in a 1/60" twist with shallow and wide grooves in the Forsyth style.

I haven't yet run either of these rifles over the full range of the express sights, but hopefully I will in the next couple of months.
That's where I'll know for sure how the loads are doing down range, and how close to the original regulation load I've gotten with my handloads.
My guess is that I'm pretty close.

Cheers
Tinker