ok, simple terms... my theory... the pellet doesn't accelerate after leaving the barrel, strippers and silencers dont make guns more efficient... what a further gap between chrono and gun, or having a stripper/silencer does, is remove a pressure wave which distorts light which makes the chrono see the pellet in the wrong place when it's measured, which gives a lower speed reading than it should if this pressure wave is still present.
the higher power the gun, the stronger the bang and thus sound pressure wave leaving in front of the pellet (assuming pellet is subsonic) and the more this upsets the chrono, requiring a greater distance between gun and chrono to allow this wave to dissapate.
In short, the pellet isnt going faster, the chrono is just seeing it in the wrong place and measuring it wrong.
this is the effect of a pressure wave below, subsonic... see how the boats and houses above and below the words US navy are distorted and not in the exact right place. It is more apparent in direct sunlight (another reason why direct sunlight might screw up chronos). However the effect still takes place even in diffuse light. Whether infra red is affected as much I don't know. Notice how further away, the wave has lost it's energy and the picture distortion is far less. Just below the fuselage, to the right hand edge of the picture, close to the plane, the distortion is massive. Although these aren't sound waves per se, they show what pressure waves can do to light.
The pressure wave we hear as sound... it's not the compressed air behind the shot, that travels slower and within this pressure wave, which is like an expanding ball. Supersonic items overtake and puncture the surface of this pressure wave, leaving it behind, which is why supersonic rounds get to target before the muzzle report is heard, although you will hear the sonic crack a split second after the round has passed, the muzzle report could be some time behind.
or this... albeit a rather big bang... the pressure/sound wave can be seen distorting the view of the shoreline about a 1/4 in and 3/4 across the image, putting it in the wrong place.