We are about building cannons here, but we only do it one way, the right way. There is a sticky at the top of the board about cannon construction. Read that. Follow those guidelines to make your cannon. We do not differentiate between a blank fire cannon and a projectile firing cannon. Two reasons. First pressure. Pressure is what makes the loud bang of a blank round and pressure is what makes the ball fly. If you don't build the gun right, pressure can get you. The second reason we don't differentiate is human nature. If you build a "noise maker" sooner or later someone is going to stick a ball in the bore, like I said, human nature. You should especially concerned if you park your cannon by your flag pole. Some Prankster or thief may come along and create a disaster.
Your Idea is doable, but just not the way you are suggesting. Bowling ball mortars are often made with an old Argon bottle.
Here is drawing for making a chamber for a Mortar.
The outer diameter of the chamber piece is three times the diameter of the chamber. For a cannon you just make a longer barrel/chamber, full length of pressure bottle.
You can make a round plate the diameter of the bottle with a hole in the middle for the barrel to cover the muzzle. Get some pipe, inside diameter of the outside diameter of the pressure bottle. Cut off a 1 inch section of the pipe and weld it on the muzzle to make fake reinforcing band. Cut another section 4 to 6 inches long and weld it on the back of the barrel to look like a breech ring. Use a trailer hitch ball for a knob. Trunnion should the same diameter and length as your bore.
Take a look at our sticky on making blank rounds. We suggest flour as a filler. Things like grass clippings, wet newspaper and bread tend to fly out like solid projectiles. Dry newspaper starts fires.
Good luck. Post pictures as you build and we will help you. We may be rather frank about some things but it will always if you are compromising safety.
Take your time ask for help, we will gladly give. You will find you will get several different ideas from different people on how do something, that is good. It gives you something to think about.
Did I say ask questions. A saying I use to use a lot back in my working days, was the only dumb question, is the one you don't ask. So ask questions.