I think it isn't quite that simple. I suspect the 81% is more based on 81% don't think a ban is needed. There is a difference versus actually being opposed.
I think the amount is purely because of the power of social media. The government tried to push their agenda using 2013 as a magic date and that the 50% of guns are domestically sourced. Once this was debunked it was much quicker to get out to people because of Facebook, etc. Plus educating the public who don't have any interest in guns on what the exact gun regulations have helped. Hell the existing restrictions on tracking handguns probably helped because of the fact they are tracked. I would almost guess that if we didn't have that, we wouldn't have the ammo (pun intended) to back our arguments.