Romney was not exactly the poster child for gun rights. The benefits of voting for Romney with respect to guns were slim enough, or vague enough, that many people who may have voted single issue with a strong pro gun candidate, didn't.
Yes, you are correct to an extent and while I was no big fan of Mr. Romney I dare say if he had been elected he would not be pushing gun control as aggressively as Mr. Obama is.
While obviously we’ll never know Obama has always had a very strong anti-gun agenda based on his history. The NRA detailed extensively before the election what would happen if he were re-elected and many on this site accused them of fear mongering. Well, guess what? The NRA was correct.