Time to live up to my namesake.
This could theoretically work for paintball, and we already know this law works with homemade Nerf blasters, when the inner piston diameter of 1.36 ID PVC plunger tube decreases to 0.485 ID CPVC barrels. On the topic of using this in a Nerf mod for a blaster with AR's this MIGHT work by giving the flow a faster INITIAL velocity but as soon as the effective diameter of the pipe jumps back up to the diameter of your dart at the barrel, I feel as though this would not be very effective. The initial puff of air would have a higher velocity but a lower pressure. That being said, I always kept my dart posts in my Mavericks and found them to perform slightly better when they weren't drilled out.
Here's the basic equation:
Note this is for INCOMPRESSIBLE fluid and we'll get to that after these messages. But just read the equation. Basically, as the change in pressure gets larger, your velocity at the barrel gets larger. So if you decrease the constricted tube's diameter (and thereby decrease the pressure) the velocity in the constricted tube will increase. Your air will be traveling faster initially SO LONG AS THE DENSITY REMAINS CONSTANT which brings me to my next point, COMPRESSIBILITY.
Compressibility is a bitch. Just like a sponge can be squished and change it's density, so can a fluid, and all these easy equations go out the window. For a fluid to reach the point of compressibility, the air flowing in the pipe must be greater than 0.3. If the barrel is plugged by a dart, logically, the air flow cannot exceed the speed of the dart. In something like a suped up 4B, the FPS can reach 300 if you plug the pump, it's more like 250 for high end springers. So with the speed of sound at sea level being 1125 FPS, the mach number of a springer's barrel would be 0.22 and for a 4B, more like 0.27, so we get pretty close to compressibility being an issue, but dodged that bullet for now, so that above equation is valid. Honestly in some of these looser fit 4Bs I wouldn't be surprised if, when hooked up to the right equipment, we actually got into the realm of compressibility.
BUT if that restriction is made too small you begin to run into the problem of compressibility in homemades. For Nerf brand shit you won't have to worry about that.
Turbulent flow may also be something to consider, however, as many little holes expanding into a larger DIA pipe is bound to create some kind of nonlaminar flow.
which has a whole slough of problems with it too. Long story short you want laminar flow if you want consistent performance, turbulent flow increases pressure and therefore less velocity based on that shit we just read.
EDIT: As for the holes being on the outside, I suppose it has something to do with the nature of flows in pipes:
Basically, those green arrows represent velocity. So in general, the fluid velocity is higher at the center where there is no friction from the walls of the pipe. The velocity at the pipe walls must be zero (no-slip condition) so near the walls the velocity is less than at the center. If the flow is closer to the walls, perhaps this is an attempt to defeat the friction at the walls and make a more "flat" (consistent) and less parabolic velocity profile. Turbulent flow would also accomplish this as the velocity profile of a turbulent flow is sort of plateau shaped and very flat like the second set of green arrows. I'm not a paintballer, so I really couldn't tell you the reason they use it, but this is just my educated guess.
So is this method worth it? Eh, I'm skeptical because as soon as it reaches the dart the DIA will increase therefore decreasing velocity, but for the instant the dart is pushed right up against the restricted region, the velocity may have some added effect, so it may be more effective in airguns where pressure is released in one instant as opposed to a springers which build up pressure as the plunger moves forward. If anything this may just reduce deadspace and that's where the increase in (apparent) performance comes from. I'd say test it out and experiment a little, and look what other kinds of experimentation has been done, i.e. potato gunners and the like probably have some insight. In the words of Ms. Frizzle: Fuckin' go for it.
Resources for nerds:
Khan Academy: https://www.khanacad...and-pitot-tubes
Venturi Effect: https://en.wikipedia.../Venturi_effect
Choked Flow: https://en.wikipedia...iki/Choked_flow
Compressible Fluids: https://en.wikipedia...mpressible_flow
Edited by Aeromech, 03 January 2016 - 11:27 PM.