The answer for about one in four Americans seems to be the Second Amendment. According to a recent Gallup Poll, about 26 % of Americans support a ban on handguns.
More here. (H/T NRA ILA)
UPDATE 2–After a second look at the data in the post at the Gallup site, I believe my one in four figure for folks who don’t understand the Second Amendment may be optimistic. Almost half favor restrictions on semiautomatic rifles.