The only thing I question is the retention of the front O-ring on the valve piston.... You might be better to use a conventional poppet and seat arrangement at that point....
Cartridges are fine, but by definition are dump valves with limited volume.... That limits both the FPE and efficiency.... They have their place, providing your expectations are realistic...
With a conventional timed PCP valve, and let's say a reservoir of 10 times the barrel volume (quite common), starting at 3000 psi, if you keep the valve open until the pellet reaches halfway to the muzzle (which releases the same amount of air as your dump valve contained), and close it at exactly that moment, the pressure profile would be 3000 psi at the start, 10/10.5 x 3000 = 2857 psi at valve close (compared to 1500 for the dump valve at half distance), and 2857/2 = 1429 psi at the muzzle (compared to 1000 for the dump valve)…
I don't have all the numbers in front of me, but one of the problems with the MAC cartridges is that the internal volume was insufficient compared to the barrel volume of the large calibers (eg. .357) they were trying to make work, and in addition the internal ports were too small.... barely OK for a .25 cal.... I can tell you that the internal volume was only 8 cc.... This severely handicapped the performance in the larger calibers.... They weren't even a decent performer in .257....
Lloyd's Internal PCP Ballistics spreadsheet
I'm not trying to convince you that you can't build a dump valve that will work and provide good performance, far from it.... I'm just trying to point out some of the pitfalls along the way to consider, and build around, in your design.... Some people won't care about the efficiency, figuring "air is free", and I can understand that.... Others, such as those hand pumping, will have a completely different attitude to keeping the valve open after Elvis has left the building....
Lloyd's spreadsheet is proprietary, and he has only given out a couple of copies of it.... I was one of the fortunate few, and of course cannot share it further.... Sorry....
- Valve configuration and opening times. In fact, GGDT models four different types of valve: chamber sealing pilot, barrel sealing pilot, burst disc, and "generic." Each of these valves have different behaviors and GGDT accounts for these behaviors (more on that below).- Pressure drop across the valve oriface.- Temperature (and thus pressure) increase in the valve pilot due to work performed by gun gases on the valve piston/diaphragm.- Gas leakage from the main valve body into the upper valve chamber (pilot).- Performance differences due to different gases.- Temperature effects on gas properties (and thus, performance).- Performance limitations due to flow choking in the valve or the barrel.- Valve effective oriface increases due to lowered valve throat Mach number.- Temperature (and thus pressure) drop in the barrel due to work performed by the gas accelerating the projectile.- Gas leakage around the projectile in the barrel.- Compressibility (Mach) effects on air pressure both in front of and behind the projectile to include the creation of shocks.
My guess is that some of the GGDT spreadsheet does not apply well to our very "small" caliber spud guns.... but then that is not what it was designed for.... In some of our highest powered PCPs, the air has more mass than the bullet....
interestin approach using ballbearings for locking the valve.
Using steel rod for the valve stem will give you some more volume.
MAC cartridge was doomed from the start, they made a cool concept but the cartridge valve was very poorly designed, very inefficient, two small holes to feed the barrel, when it could have dumped like the Lane cardridge from the case mouth.
Very Cool.... Bob