Each caliber comes with many possibilities for speeds. And quite a few different projectile profiles due to the different manufactures involved with the B/P sport. There's the issue of different brands of powders and grades and their charge rates up-down a volume measurer having to deal with.
My suggestion would be be more specific OP: Kind of projectile, (ball, bullet, sabot, maxi or mini) its weight, type & brand of powder used. Asking a question in that manner would narrow the field substantially. All the years of shooting I have never seen a fellow enthusiast chronograph their B/P rifle. Saying that. Only applies in my very limited experience and not meant that it has never happen. I'm quite sure it has been done by someone.
Watch it!!! their both out to get us.