Bk rush hits a bit stiffer. I would think thats due to the taper rise being a little different.
Also, the bk rush is 12.75 or .9...if so, the feel would be close if you play with the 12.9 but, it wouldn't be if you use the 12.4.
I've used a bk rush with a one piece leather tip and IMO, it is way different than the 12.4 playing shaft.
YMMV,
Jeff