Designing a better flowing valve

[rsterne]

While a properly designed C-D nozzle can accelerate the gas itself past Mach 1, the problem is that when the flow chokes, the pressure is reduced by 47%.... This leaves less force to accelerate the projectile....

Bob

[MJP]

I tested De Laval nozzles in the past, with very light projectiles you would gain some speed but anything heavier than plastic pellet would loose velocity.

Like Bob said you loose too much pressure and the flow is not fast enough over the nozzle to gain anything.
On open nozzle the thrust is bigger but then you have flow velocity gained from the nozzle.

Marko