I've always thought it was bollox as to a pellet accelerating after it's left the barrel, all my instincts said so, and i reckoned the physics did, but wasn't good enough in that dept to prove it.
About a year ago i saw a strange phenomenon following a slow moving jet aircraft in direct sunlight, the camera captured what looked like a split wing. It wasn't a camera or shutter problem, and the "split" moved up the wing quite quickly. Posting it on a photographt forum revealed a much more severe version with an sea vixen with it's rear wing litterally severed by the effect. It can be seen here...
and the other example is attached on this thread.
it now seems as if that's a vortex of different pressure air created by the aircraft, which are moving much slower than supersonic... and this causes an index of refraction shift... (index of refraction being the amount light is bent in a material... the reason why the straw seems to bend when you look at it sitting in a glass of water at the surface)
i know from shadowgraph photos of subsonic ammo, that low speed projectiles also have a pressure wave in front, caused by the sound at the muzzle... this travels at the speed of sound, and is the noise you hear.
as this is sound wave is an intense change in pressure, i theorised it would cause an index of refraction change that could be severe enough to appear solid to a sensor on a chrono... or different enough from still air to appear so.
it would appear pyramid air know about this, or have come to the same conclusion... (found this on http://www.airrifle.co.za/
If we had a super-fast video camera filming the muzzle of a spring gun, you would be able to see a ball of pressurized air that comes out of the gun ahead of the pellet. If the pellet travels at 900 f.p.s., this ball of air goes about 1100 f.p.s. for a couple inches. If the muzzle is held too close, the skyscreen senses the ball of compressed air and starts the clock. Once the clock has been started the pellet has no effect on it anymore. Of course, the pellet passing over the stop screen stops the clock and now you have a longer interval on the clock than the pellet really should have registered. More time equals a slower pellet transit time, hence the readings are slower than they should have been. Just by backing the muzzle up 12 inches from the start screen, you take care of 100 percent of this problem with all spring guns. Maybe with some powerful pneumatics like the Condor you should back up 18 inches. And certainly with a big bore I would back up 4-5 feet. The pressurized air will still be seen by the skyscreen, but by backing up you allow the pellet/bullet to trip the sensor first.
So, by moving the chrono further away, this pressure wave disperses and you get a truer but higher reading... same with silencers or strippers that either disturb or divert this pressure wave away... although strippers/flippers and brakes don't remove the sound, a lot of this is blasted back or away from the pellet... thus removing a lot of the pressure wave, and thus seemingly giving a power increase.
So, i think the jetting theory is even more bollox now.
While im on it, i'm convinced that strippers etc only apparently improve accuracy by reducing recoil and vibration and allowing better follow through, but that the removal of this turbulent air front and behind does nothing for the actual accuracy of the pellet in flight... the wave in front is gone, as it travels 300 fps faster, and the one behind will probably always be there due to the low pressure formed by the pellet moving forward.
Of course, when someone says something that agrees with your own theory, it doesn't always make it right... could just mean there's two nutters in the room...