I totally understand the doubt on the muzzle velocity. I was freaked out when I first saw those numbers.
Here is my understanding on what is going on (I have gone over this with a number of individuals specializing in areas of concern here).
The way this began was simple experimentation with PVC parts and a sprinkler valve, as many others have begun. I began experimenting with other valves (that is how I moved toward brass bodied Granzow valves). I began running larger bore valves with barrels that tapered the diameter down to the 1/2 inch barrel. The best results were from a 1" bore valve tapered to a 1/2 inch barrel. This dropped the turbulence down within the valve to sub-sonic levels. At that point, I was seeing 1,050 FPS at 200 PSI consistantly. The next experiment was in lengthening the pressure chamber. This was not for added volume, but as a means to trick the PSI a touch higher. Air weighs 4.7kg (If I remember correctly) per cubic yard. If that air is rushing forward toward the valve, you can achieve an increase in pressure behind the projectile. This is from the smaller diameter but longer chamber creating a higher forward speed of the air (still sub-sonic) toward the valve. The innertia of the air, along with the pressure gave a HUGE increase in muzzle velocity. The problem is, this is a very sensitive ratio, apparently. I fired round after round from various distances and consistantly saw the same velocity. However, after that, I began doing more testing and every other change I made reduced velocity noticeably. So, I found a sweet-spot. However, that sweet-spot is not something I can build and carry around. The darn cannon is 8 feet long! It was merely built as a test bed to see just how fast a projectile can be pushed. Once that information was obtained, I gained a far larger understaning of what is going on with these cannons and I am using that knowledge to build a few truely useable guns. All of those will max at about 800 FPS max, however, and those are the small bore guns. I do not think I could hang onto a 2 inch gun with that much power. I am hoping for 500 fps with my 2" gun.
My primary source of information is a friend who does research (or did in college) on a supersonic wind tunnel. That tunnel is simply a huge air cannon like we use with numerous air flow stabilization steps taken. The key is to keep the flow at sub-sonic levels through the valve, then accellerate the air in the barrel only. That keeps any shockwaves inside the smooth barrel, rather than creating even more turbulence in the valve. That is the primary principal in achieveing high super-sonic air flow in these wind tunnels.
My source of air is a Helix low rider airbag suspension system compressor. I am using lithium polymer batteries to drive it (left overs from one of my E-bikes).
It is reated at 200 psi. But, I can sneak 250 out of it running at 24 volts rather than 12. It get hot that way, however.
At any rate, super-sonic cannons are not all they are cracked up to be. For one thing, the accuracy goes completely out the window. For another, the velocity drops to sub-sonic leves so rapidly, it is really not feaseable to use unless you can hit extremely high mach numbers. It is far better to stay slighly sub-sonic with the highest weight projectile possible for the intended target.
Bear in mind, I am totally open to my Chrony being wrong. I researched that as well.
I wish I had a way to calculate the pressure (FPS) required to penetrate two sheets of 3/4 inch plywood with a 9 gram .60 caliber projectile. Wether my Chrony is right or wrong, I have way more power than I can really use.
Part of me joining this forum is to learn what I can from those who are farther along the path.
Oh, lastly, when I was down in the 850 fps range, I did get one 2,150 FPS reading from my Chrony. I know that was an error. All the 1,700 FPS readings were repeated under various conditions. Again, I am open to it being wrong, however. Hey, maybe two chronographs in a row would be good for redundancy?
Matt
The kick with only 100 PSI is plenty.
