The Firearm Blog has an excellent write up from a reader that explains how Russia joining the World Trade Organization will void other trade agreements with the U.S. which currently puts heavy stipulations on imports, which is why partly Saigas have to be imported like they are. If this changes, which would still take over 3 years, we could see a big change in the surplus market again. I'm not sure though how much the ammunition market would shake up though. If Russia has a lot of steel core M43 7.62x39, that won't be able to be imported due to it being classified by the BATFE as "armor piercing pistol ammunition" from the first 7.62x39 pistols in the 90's. Also Russian 5.45x39 is already imported, I don't know if like the writer mentions there that the Russian ammunition comes from Ukraine first now or not. Also 5.45x39 versions after 7N6 I believe are already banned due to them being classified as armor piercing (but still not pistol, which is why 7N6 can be imported and no one should make 5.45x39 pistols). However surplus ammunition like 7.62x54R, non steel core 7.62x39 (direct surplus 8M3 anybody?), and 9x18 could be imported and maybe another source of 5.45x39 could be added. Saiga's may still be imported the way they are mostly due to 922R, but Russian parts kits may become more available as well. The biggest change would come in the parts and accessories market. These could flow a lot more freely.
The other thing to keep in mind though is that regulations could still be added back in or bureaucratic rulings come down that it isn't acceptable to be imported. A similar example to that is the M1 Carbines that the current U.S. State Department won't allow to be given back to the U.S. by South Korea. I generally don't get too political here, but want to say this is one example of how an anti-Second Amendment administration can still limit rights from the Executive branch even without control of Congress.