In fact its a guess of mine with proper porting you could run M1 06 with powders you are not supposed to, if the calcs and porting were changed.
You are correct. Adjustable plugs are available in two designs, called the Schuster and McCann plugs, that feature adjustable orifices for bleeding off excess gas pressure.
A third and interesting design is by Eric Claypool, called the Ported Plug, and uses the principle of volume of area to lessen the peak pressure but provides a larger volume to continue to operate the action. A lower peak but more time under the curve. This allows for slower powders and heavier bullets to be used.
You can read his write-up and view well-presented data on his webpage, GarandGear.
