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One has to be careful there. Yes, according to that data, in Italy 67% of homicides are by firearm while 11.9% of people have a firearm. Since the numbers for the US are 60% and 89% one is tempted to conclude that in Italy there are more firearm related homicides. However, this is false. If one looks at the fourth column it is clear that in Italy there are 0.71 homicides by firearm for each 100k peoples while in the US there are 2.97, more than 4 times as many.

Brasil on the other hand has indeed much more homicides per 100k people than the US, although having only an 8% of the population legally possessing a weapon.

TL;DR: In Italy there are four times less homicides by firearm per 100k people than in the US.