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.
"JUST A OLD DEPLORABLE THAT'S IRREDEMABLE."