Those familiar with spudguns know that a volumetric propane meter typically uses two ball valves connected to a regulated propane supply. The first valve lets propane into a small reservoir of specific volume. That valve is then closed off and the second one is opened, allowing the propane inside the small reservoir to enter the combustion chamber to mix with fresh air inside. The spudgun is then ready to fire. My test rig does essentially the same thing, but consolidates the metering system into one little 3-way valve with some convenient features. It has a threaded portion below the button and it exhausts air (in this case propane) around the button. This valve is mounted at the back of the combustion chamber using those threads such that the button is inside the chamber. The test rig has a floating barrel portion/piston with a rubber seal. When the barrel is pushed back within the combustion chamber, it impacts the valve's button and causes regulated propane to enter a tiny reservoir. When the barrel is pulled forward again, the valve then releases that propane into the chamber to mix with fresh air that enters via a check valve and aided by vacuum caused when the barrel is pulled forward (assuming it has been loaded with a dart). The rig is then ready to fire. The spark is provided by a piezo igniter.
The video should explain this process more clearly: http://s237.photobuc...nt=MVI_2467.mp4
Note: That messy looking pipe nipple sticking out from the valve is NOT the reservoir. The schrader valve is not part of this either. I just put those on there to protect the ports from epoxy when I sealed everything off. The actual reservoir exists between the perpendicular port on the valve, and a hex plug that fits flush inside it. The reservoir is ridiculously small in reality. Propane is injected at 45psi. So far the rig has fired several hundred shots and the propane tank is still quite heavy.
(Actual fittings shown here)
It should be obvious how the firing cycle can be adapted to a bolt-action configuration with that barrel/piston plugging into a barrel extension on the forward stroke and exposing a breech on the backward stroke. When I have time, I will complete the rig and post more pictures. With an effective barrel length of only 3 inches, the rig gets consistent ranges of about 60 feet held horizontally four feet from the ground.
DISCLAIMER: This project involves PROPANE, HIGH VOLTAGE, and FIRE! Be aware of all risks involved when starting a project and DO NOT hold others [ME] responsible for whatever bad things may happen.
Edited by PVC Arsenal 17, 18 July 2011 - 04:11 PM.