The real issue here is there any reasonable way to ignite smaller cannon with close to scale vents. It's true, the toy cannon you sent us the plans for would be a good candidate for this small diameter vent ignition method, but so would thousands of others out there like the small brass guns and small steel tubes we see being made quite frequently. If some owners of smaller cannon would cover their vents with a piece of tape with a 1/16" hole in it, or maybe a 3/32" hole, Mike and I are quite certain that the "closer-to-scale" appearance would pleasantly surprise them. An added benefit of a small vent which we take very seriously is the increased retention of pressure per any given quantity of powder.
We have considered interchangeable vents, but we work very carefully to get a copper vent piece which does not leak any gas and if you screw these in and out a few times, they will start to leak. Also this switching vent pieces won't work for the vast majority of small vents which are simply drilled holes in the brass or steel of the tube. We do all the broom straw cutting at Mike's house where our shop is located. As he is not currently married he can pretty much clean carburator parts or cut broomstraw on the kitchen table without offending anyone. Mike's friend Bruce had 80 pounds of suspension parts for his '48 Willey's that he is rebuilding on that table and no one was offended, not even Gary, AKA RocklockI, and he offends real easy! So, everyone's situation is a little different and as for the SWMBO rule, I think I violated that one in my own driveway back in '89 when I set off a whole lunch bag of old gunpowder one 4th of July. I never heard this end of that one! My assertion that it was a mere swoosh, not a bang, didn't help at all. I was smart enough to get up at the crack of dawn the next day to scrub that huge black mark off the drive.
I might just build that cannon I now have plans for, thank you, but it's going to have to wait awhile as I have way too much on my plate right now.
Thanks for sharing your ideas!
Tracy and Mike