My non-scientific study has shown that my FSR shots are more accurate with the Hammerhead reverse ported muzzle on my Bangstixx than without.
So it's probably true that exist gas is impacting the FSR when exiting the barrel.
Based on my personal experience. I have had a few pictures of me shooting FSR and in each shot the FSR exits the barrel before the cloud. That is with a 16 inch barrel at 400psi. Dunno what the behavior would be with a longer barrel at higher or lower PSI since the air volume would be different.
I like Spyders.
MR1, MR2 LP, EMR5 LP, EM-1, Raven Primal, Victor, Fenix.
Magfed Hurricane with T15 Mags