doing a Ladder test

[fiver]

something is going with that.^^
IMO it's more the speed of powder kernels coming out the muzzle exceeding the speed of sound, even from a suppressor.
I don't think they are being pushed from behind but being sucked from the front and they don't have the oxygen to burn properly until they hit the air.

 

[popper]

That was the assumed answer. SEE is caused by collision of burning powder wave and slower muzzle gas wave.