In 2006, there were 400,000 violent crimes commited with a firearm. With a populace of 350,000,000 people, that's about 1 violent crime for every 800 people. That is total violent crimes involving guns, not just homicides which will be even lower. The UK actually has a higher violent crime rate and those homicide stats include a lot of non-gun realted homicides (which include the most common one, vehicular manslaughter, and the second most common domestic violence - a crime usually commited with ones bare hands or household utensils like kitchen knives).
So compare apples to apples: gun-related homicides between nations, and we don't see near the disparity onw would expect. Next, compare those gun-related homicides only with legal and valid firearms. The number drops dowen to damn near nill. Illegal guns exist even where there is the strictest gun control laws and, in fact, more exist per capita where guns are outlawed, hence the saying if you outlaw guns, only outlaws will have guns.