It's a gun-off. From McCain's statement on Heller:
Unlike Senator Obama, who refused to join me in signing a bipartisan amicus brief, I was pleased to express my support and call for the ruling issued today. Today’s ruling in District of Columbia v. Heller makes clear that other municipalities like Chicago that have banned handguns have infringed on the constitutional rights of Americans. Unlike the elitist view that believes Americans cling to guns out of bitterness, today’s ruling recognizes that gun ownership is a fundamental right sacred, just as the right to free speech and assembly.
Sen. McCain has not advocated an absolute prohibition, “but he cosponsored legislation which could require registration of attendees at gun shows and even ban such shows,” Barr warns. And Sen. McCain’s campaign legislation “curtailed the First Amendment right of gun owners to protect their rights by participating in election campaigns.”