Before answering guys, please note that one of the 19th century muzzleloading experts - Gibbs or Metford, I think - went to the trouble of suspending (NOT clamping) a rifle on an axis that was true to the barrel and rotating it. He fired shots through all 360 degrees of rotation. Rifles that shot high left shot in the normal position shot low left if turned to the left, low right if turned upside down, high right if turned to the right etc. Check it out!
Patrick.
P.S: A detailed treatment can be found in "Geschosse",
P.P.S: For those who may not have heard of him, Beat Kneubuehl is THE
Swiss
ballistics expert. No-one needs to trust what I say, but if you want to say he is wrong, you need a lot more than anecdotal evidence.