From some bits of info here and there, you should follow up with looking further into the comments that velocity reducers do work for semi-auto shots but fail to work for more than the first couple of shots of full auto.
Something to do with full auto disrupting the 'baffling' enough to negate it's effect...
Sorry, no personal experience to back this up...I prefer to just set my rifle around 380-ish and be done with it.
If your field allows such high (i.e. 460-ish) velocities...then why do you want to lower it? If they don't (i.e. they max out at 400) then why wouldn't you forgo all of this fiddling around and just drop the FPS down permanently? If you're hitting multiple fields with different limits...setup for the lower one and don't sweat it.
You said that you're rifle is "most likely" shooting 460 fps with 0.20g bbs...well, it either is, or it isn't. Measuring it's velocity with a chronograph is the only real way to tell for sure...everything else is just guessing around.
Best of luck!