It could take on many forms, and before someone says it "What is an assault rifle"?
They could ban components like they did last time (collapsible stocks come to mind immediately because I could not get one).
They could also go after the sales of full rifles that meet a certain criteria to be sold after a certain date.
Things like uppers for an AR would be harder because you don't have to have any FFL involved, so no paperwork unless the seller hands it over.
With components they can simply see it on your rifle when you go to shoot. There are limited ways to look at an upper and say "That is a post ban upper, get over here".
If you have a receiver, of any type, on the books prior to an AWB going into place, I would say you can probably assume that there must be a grandfather clause. If there is no grand father clause, then I suspect there will be one final showdown from all those that don't want to give up what they paid good money for and have a legal right to under those ones called "Inalienable".