Interesting question AI.
I have a BK9, but am using the kydex sheath from a Becker Brute, so can't
help much, but this weekend I'll see how it fits in that sheath just out
of curiosity.
My brother has the original BK9 sheath (we swapped because he liked the
cordura sheath) so I'll ask him to bring that in to work on Monday and I'll
check that out for you as well.

Cheers