I can understand why America would need to keep guns legal - after all, with your current availability of guns and how happy criminals are to use them, the immediate banning of guns may make things worse, not better. In any other situation, the argument would be pretty weak, after all, whilst it's not impossible for a criminal to get a hold of a gun when they're illegal, but in countries where it IS illegal, like the UK you can actually see that far fewer murders involving firearms are happening and the really US suffers for gun-related murder and I imagine it's a very difficult issue to solve.
To put it into perspective, for the UK to match America's gun related murders, you'd have to multiply the current statistic by ~624 (UK - 15, USA - 9,369, as I posted above). Now, I don't have per capita, but considering the US population is only ~5 times that of the UK (Population - 61,414,062 (UK), 307,006,550 (US)), then it won't take a genius to work out that the rate in the UK per capita is a LOT lower than that of the US. We probably could compare with other countries too, but I'd argue that maybe a society where the public doesn't have easy and legal access to a personal armoury is safer, but of course, there's a number of factors that affects statistics, you can't place problems solely down to gun use and I think there's a lot any country can do to keep its crime down and there are many reasons why countries like the US and UK suffer in crime figures (UK still suffers, whilst it's not as big for violent crime) whilst the likes of Iceland almost seem crime free, so there's plenty of other things that need to be considered, such factors might help the US where the banning of guns wouldn't.