To answer your question a bunch of milsurps were banned. All anti-tank guns were (Boys, PTRS, PTRD, Solothurn 20mm, etc.). All items with a bore over 20mm were banned as well, many cannons, mortars, and some antique firearms fit in that category.
As to what they are likely to do next, that's anyone's guess and you honestly cannot predict it unless your in their war-room. They might push for more, they might be content for the moment, they might do stuff which doesn't actually ban certain firearms but makes it extremely difficult to possess them (such as banning owning more than 5 guns, if you want to see market price plummet that's how you do it).
Basically buy what you like and hope for the best. You can take more reasonable guesses as to what will be safer, but that is no guarantee. I would never have guessed a ban on anti-tank rifles was coming, and lots of people had a lot of money tied up in them. At one point I even considered building a AT gun collection though cost kept me out of it.