Summary

I made a large double-walled 2-inch barrel cannon similar to that laid out by "The King of Random" https://youtu.be/Zuc3Oc3r7ww
since then I have printed dozens of these 50mm FFR's at about 51.5mm scaled diameter and several lengths. I prefer them printed in TPU/Flex material because they last many many launches, I have lost one at over 250 yards at 50 PSI with a tailwind of 12 mph and I am continuing to modify them, I wanted a little more twist, and a little better stabilization in an ongoing search for further distance, and more fun.
I have printed them with a single exterior wall, and concentric infill at 15% from TPU for a very soft and spongy projectile that doesn't last more than 12 launches, but I have some that have seen more than 50 launches and are my goto, for long-range and lots of running.
Currently, I am waiting on TPU, and printing from my reserve supply (3d Solutech: Carbon Fiber reinforced PLA, with a 0.8mm nozzle, 30mm/s, 0.2mm layer height, 215C hot end, and 45C bed, on a DIY Prusa I3, that is 4 yrs old.
This thing looks SICK! I can't wait to give it a test flight after dinner, and see how she flies!
If you print, build, and launch, I hope you are aware enough that this may go over 300yrds or about 300 meters, depending on many variables, it will also have a lot of energy to dump, even at that range. So calibrate your scaling of this model, to the printing of your printer, and the diameter of your launcher, it should be scalable down to about 25% which is a half-inch, even with a 0.8mm nozzle.
The post in the bottom/back has a model diameter of 22mm, with an overall diameter of 51mm, this may or may not need to be scaled depending on the settings, precision, and needs of your setup and technical understanding of your equipment.
As always, have fun, and live life.
Update 5 Aug 2020, thinner vanes, much less twist, not scalable down. we opted for a Howitzer style cannon, instead of mini cannons, with this, we will have a coaxial pneumatic system(4 inch exterior and 2 inch interior barrel), and a piston valve released by a ball valve, to a 3/4 inch opening, so I am sacrificing my small cannon, to make this 10 foot long (about 3 meter) monster happen. our objective is 1000 feet, or about 0.3 KM. then the boys will have to track it down.