Steam Cannon

• The Mythbusters designhad a very long cold barrel. Even if the initial burst of steam was enough to move the ball, the steam would have condensed as it traveled up the cold barrel. The condensing steam would have created suction equivalent to the original burst canceling it out. In the show you could see liquid water pouring/dripping from the tip of the barrel, A sure sign that the barrel was not hot all the way to the tip. The reason is that iron is a fairly poor conductor of heat. You can heat an iron bar to glowing red on one end and still be able to touch it at the other. The outside of the barrel needs to be wrapped in insulation. Without insulation they couldn't even get a basic boiler to work. Same principle here, if it's not hot it's not going to work. To improve the design even more wrap a layer of copper or other heat transfer material inside the insulation against the steal pipe. This will make sure you have a barrel with an even heat.
  

• They could try different mixes of water from pure distilled water to heavy salt. Distilled water should turn to steam faster and at a lower temperature. Salted water needs a higher temperature but may result in a bigger boom! You could even try an alcohol/water mix, but only use enough alcohol so that the volume of your water is not impacted.

• They used a large steel cannon ball designed for a much more modern gun powder powered cannon. Two problems with this:

1. The steel ball is very heavy. Could be the gun fired something lighter such as a ball made of oak or many small oak balls (grapeshot). The claim is that the gun worked. There was no comment about what it was used for. Did it fire at ships or people? Was it a scientific curiosity or was it an effective weapon? We don't know. We don't even know if it was mounted outside to fire at on coming enemy, or inside at the end of a long hall as a last defense.
2. The gun barrel was sized for a modern cannon ball. Could be the gun used a smaller dimeter barrel. If the hole is half the diameter then the ball should have twice the pressure. A quarter the diameter, four times the pressure...

• If all else fails we KNOW the gun will work if it has a valve. Add a simple pressure valve that automatically releases when theres enough pressure. Theres a few different ways this could have been made at that time:
• Hammer a wooden spike into the end of the barrel. As long as you don't hammer the spike in too much the spike should fail before the barrel does. You don't need ALL the pressure behind the ball, just most of it. This solution would pressurize the barrel behind and in front of the ball. Just make sure you have enough volume behind the ball to give it thrust. This solution has the added benefit of heating the barrel from the inside making external heat distribution unnecessary (insulation would still be a great idea though the steal barrel itself may give enough insulation)
  
• or you could insert a thin sheet of copper between the ball and the steam. Copper is soft (especially when hot) and would fail before the steal did. Two ways I could see this working. 1) cut the back off the barrel, add a sheet of copper and then clamp the barrel back together again. 2) build a small steal frame that suspends a sheet of copper. Cut a slot in the back of the barrel that this frame can be inserted into and will completely block the barrel.