Some point to the fact that guns weren't used in the London killings as evidence that if you take guns away, the criminal will simply find another weapon to use.
But, would go a step further. If guns had not been effectively banned in England in 1996, perhaps they would be used in more mass killings today. By banning the gun, Britain took away a weapon that can kill a lot of people quickly.
Knives are not as effective. The mass murderers are being relegated to a weapon that reaps fewer deaths. If the London thugs had had easier access to guns, would they have chosen to use them? And, would we be looking at more people having been killed?