Well, you can buy post-ban ones that have fake flashhiders, or no flashhider and full A1/A2 stocks. They can have muzzle breaks that are pinned, as someone above said. Buuuuuut, you can buy uppers that are pre-ban. It is up to you to stay legal at that point, but isn't that always how it works? I mean, you *could* buy a post-ban and put a telescoping stock on it, but once again, you'd be breaking the law. Basically, you can get any of the rifles that Bushmaster sells in a post-ban configuration, with A1/A2 stock, and no flashhider/bayo lug. You can get a threaded barrel, but it is up to you to have your fake flashhider or muzzle break properly pinned and welded on.
Also, if you want any of the strange calibers (such as RRA), you must keep in mind, while you can get a post-ban rifle with such a caliber, it may be very difficult or impossible to get magazines for them. For instance, if I want a RRA in .458 SOCOM or 6.8 SPC, I can get an upper-receiver no problem, but I can't get any magazines due to capacity issues.