Having seen a good number of folks come here looking for advice and having based on that experience a pretty good idea what your are thinking based on what others have asked I suggest you start at the top of our forum and reading the stickies. There is lots of good stuff there but the place to start is with the safe loads and cannon plans. Lots of good design info in that sticky's
The cylinder or tube of most hydraulic units, by them self do not have thick enough walls to be safely made into a cannon. The rule of thumb for cannons is the wall over the chamber area should be equal in thickness to the diameter of the bore. If you can find a cylinder with thick enough walls, it might be a good place to start.
You are going to need to make a breech plug and fit it to the tube. This breech plug if made with a howitzer chamber would let use a thinner walled cylinder for the barrel. You will find links that describe how to make and install a safe breech plug in the safe load and cannon plans sticky.
Something we preach here, yes preach..do not build you cannon for what you will shoot in it, build for it for what the next guy to own it will shoot in in it. We don't build bombs we build cannons. Even blanks can blow up a cannon.
You could also contact our sponsor and see if they will build you a tube. You might find it cheaper in the long run.
So welcome to the board! Get busy and start building, we are waiting to se the smoke and fire!!
Semper fi.