I've never built one, and there are probably plenty on here that have. That being said, I would use pvc pipe for the barrel. You'll need to decorate it with paper mache or foam to give the outside the look of a canon. I would hook a fog machine at the back end and use an air canon to blow it out. I would also insert an orange light in there. You can time it all, along with sound, by using a controller.
You may be more skilled than me and build your own controllers, but this year I'm using a couple of Nerve Center controllers from Monster Guts Prop Controllers - Nerve Center - Halloween Prop Building Supplies For The Home Haunter - Monster Guts
and various picoboos from Fright Ideas
Controllers : Fright Ideas, Control the Fear
You may be more skilled than me and build your own controllers, but this year I'm using a couple of Nerve Center controllers from Monster Guts Prop Controllers - Nerve Center - Halloween Prop Building Supplies For The Home Haunter - Monster Guts
and various picoboos from Fright Ideas
Controllers : Fright Ideas, Control the Fear