Breech port theory for spudguns
This all stared with my new coaxial breechloader. This first image provides a sketch of this. After building this gun, I looked at the gun and I wondered if the breech was too close to the endcap, which could partially choke the flow. I know there are several good solutions to this, such as:
1. cutting the breech of the barrel down, or
2. extending the chamber
But I also wondered about drilling holes into the area where the spud is loaded. You can see this in the next sketch. The arrows represent hole locations. I figure this might help if the flow were TRULY choked at the rear of the barrel, because as the spud begins to slide forward, the barrel ports (holes drilled around the circumference) would allow more pressurized gas to enter the barrel. If it works, this might have an effect similar to a burst disk, by allowing slightly more time for pressure to build, and thus would most likely benefit launchers with a high C:B ratio (such as my 1.5:1), and in which folks don't want an unwieldy 10 foot long barrel to utilize all of the available energy.
The key is to initially restrict the flow, and then to allow the full flow.
But truly, I DON'T think these ports will help much in my gun b/c I DON'T think the flow is REALLY choked at the breech. My observation is that this gun performs about the same as my traditional muzzle loaded spudgun, and both have similar chamber volumes and C:B ratios. Because I'm not really worried about choked flow, I don't think barrel ports will make any difference for my gun, as the barrel itself will be the limiting factor.
So this final sketch shows what I want to test. I propose to breech load the spud into a ported barrel, and then place a cap on the end of the barrel. This cap will have a hole drilled in it, such that enough pressure is allowed to pass through in order to overcome the static friction of the spud (plus a little extra, in case of a tight spud load). Then, as the spud begins to slide down the barrel, the ports are opened and the barrel and spud get the full force of the pressure generated by combustion.
The ports could even be drilled in order to provide more potential flow than the barrel can accommodate (perhaps 110%), just to be sure all of the energy will be able to enter the barrel and propel the spud. I also have wondered about making several rows of barrel ports, with each row having a bigger bore than the previous, to provide an incremental increase in the flow.