The other day I heard something on NPR saying that, by one measure, support for gun control had declined from over 80% (I believe they said that was in the 1980s) to less than half today. I'd thought that it had been pretty steady over time, so I checked for surveys that asked comparable questions over a period of time.
There have been a number of questions on assault rifles, with the basic form "are you for or against a law which would make it illegal to manufacture, sell, or possess semi-automatic guns known as assault rifles?"
1995 68% 30%
1996 57% 42%
2000 59% 39%
2004 50% 46%
2011 43% 53%
It looks like a substantial decline in support for a ban on assault rifles. But the 1995 question had an introduction: "if you would favor or oppose the following proposal which some people have made to reduce crime: a ban on the manufacture, sale, etc." The mention of reducing crime might have made people more inclined to say they favored it. If you leave that question aside, there has been some decline in support for a ban, although not as dramatic.
There was a question in 1989 asking "do you think it should be legal or illegal to manufacture and sell semi-automatic rifles known as assault rifles"; only 20% said it should be legal and 73% said illegal. But there was one in 2006 asking if assault rifles "should be sold to the general public or should their sales be limited to the military and police"; 15% said they should be sold to the general public and 82% that their sales should be limited to the military and police. You could interpret the different results as ambivalence or confusion, or as people making a distinction (some people favoring regulation but not a complete ban).