I would say several of your points are incorrect.
A jet ignition system is extremely touchy. In research papers I've seen on the system they tend to be very inconsistent. It'll work one shot then not the next.
Most of the published work is done in a closed chamber. An operating spud gun is actually quite a bit different than a closed chamber. Movement of the gases in a gun can induce burn accelerating turbulence.
The increase in burn rate is significant but not all that
To keep the projectile from exiting before combustion is complete you can;
1. Add more spark gaps. Two gaps make the chamber burn ~2x faster.
2. Get the sparks away from the chamber's wall.
3. Increase the static friction between the ammo and barrel. For Veggie ammos this is very easily done by beveling the inside of the muzzle (or whatever is used to cut the ammo).
4. Induce turbulence with fan(s)
Fan(s) are not necessarily in the gas flow path, or at least, they don't necessarily represent a large portion of the chamber's cross section. A 3" diameter fan will probably be sufficient for a 12" diameter chamber.
Furthermore, since the fan position isn't critical for the combustion process, the fan can be almost all the way to the back of the chamber. The fan might only be 2" in front of the back of the chamber even with a 2 foot long chamber. Very little gas has to move through the area defined by the fan's blades. (That's also why a fan will usually survive in a properly designed gun, it really isn't subjected to much pressure during firing. It's probably subjected to more force after the projectile leaves the barrel and cool air is sucked back into the chamber.)
A high speed fan positioned within a couple inches of the spark gap is probably going to give exactly the same affect as the jet igniter. The high speed gas movement will be turbulent and turbulent gases burn up to 10x faster than do stationary (or laminar flow) gases.
"You can get the same performance increase, for nearly zero cost, by just increasing the size of the chamber"
Not entirely true
For all practical designs using practical materials and getting practical results, yes it is.
It is true though that an extremely large chamber might have poor performance. If the CB ratio gets up above perhaps 3 then you might
be getting into the "mine sized chamber affect" domain. I don't believe anyone has ever actually demonstrated that affect in a spud gun and, IIRC, HGDT doesn't predict a "mine sized chamber affect". HGDT predicts the performance goes asymptotic to some value.
HGDT does predict that chamber volumes bigger than CBs of perhaps 3 or so do not significantly increase the performance of the gun. Even with a burst disk (which should out perform jet ignition and is much simpler to implement and get to work) very large CB ratios don't help according to HGDT.
A perfectly tuned combustion system would be one that reaches ~120 PSIG before the projectile moves, and the chamber volume is so much bigger than the barrel volume that the entire transit is done at essentially 120 PSIG. To put it another way, 100% of combustion is done before ammo starts to move and the chamber is very big. The easiest way to realize that is to use a burst disk and large chamber. I don't see any way that a jet ignition system can get anywhere near the performance of a burst disk. Indeed, I doubt the jet ignition system will have any measurable affect on the gun's performance.
Furthermore, the orifice size and ratio of the pre- and main chambers appear to be critical to a successful jet ignition. "Successful" means the chamber actually burns faster than a single large chamber of the same total volume. Get any of the parameters wrong and there is zero benefit, and may even be a decrease
in performance. Get any of the parameters outside the critical range and the main chamber won't even ignite
A jet ignition system may well have problems with the ammo moving too soon. If the pre-burn chamber raises the pressure above the static friction then the ammo starts to move before the main chamber is ignited. If the pre-ignition chamber is big enough to actually work as a jet igniter it will probably raise the pressure enough to start the ammo moving prematurely.
A typical combustion spud gun has the ammo moving after about 10% of the chamber has been burned (CB of 0.8 or so). That means that if the pre-chamber is 10% or so (or bigger) of the main chamber then the ammo is still going to be moving before main chamber ignition.
The photo's show a gun that may well behave as a "mine sized chamber". I bet if you made the chamber smaller
the gun's performance would increase
BTW, for some nice photos of a jet ignition process see http://www.td.mw.tum.de/tum-td/de/forsc ... er/324.pdf