Well, only the "first" shot expels pressure on the port door. After that, we leave it open for the duration of that shooting iteration. I have seen residue, especially when shooting blanks, but that's the same as suppressed, the gas trap retains more.
What's isn't known is what peak pressures do exist in the bcg. It's been suggested it's as low as 1500 psi rifle, 2500 psi carbine. Where the figures are sourced is usually never given.

