I suspect that there would be a flood of NR receiver sets designed to use AR parts.
Either that or there will be a grey market in prohibited guns where all of the formerly legal gun owners carry on owning those firearms, buying and selling amongst the non compliers, trying desperately to ensure that their firearms do not get diverted to the black market where all the violent criminals do their shopping.
There already is a grey market for illegally owned guns among former FAC holders that never bothered with the PAL. It will just get a lot bigger.