This is what I love about this forum, one can ask almost any question like this and it always gets answered!
About the shells hitting the tube walls, isn't that pretty easily calculated? I could do the math, but, as always, there is always someone who have thought about this before
There you go, post no 8:
http://warbirdsforum.com/showthread.php?t=1420
but like always with physics, it's always more complicated than this... But theoretically, you'd need to pull about 80 G to make it hit the wall.
By the way, if in any post or discussion related to guns, especially aircraft guns you see the name "Tony Williams", he knows pretty much everything about the subject.