Schuster has been making those for a LONG time now and they do work pretty well.
As for not being legal, it depends on the specific competition you are referring to and which rifle/Schuster product, but they are legal for some and not for others.
I am using the Schuster bored and vented plug in my long range M1a. With a stout load of 168 Berger Hybrids the rifle runs easily and the brass is not beat up.