I think most people just say that because it's easy to repeat that mantra without actually thinking about it. The truth is there's many countries in the world that disprove a correlation between ease of attaining firearms and murder rates in those countries.
(murder rate is per 100,000 in examples below)
Venezuela - 62.0 murder rate. Before 2012 a permit was needed to purchase. Then law enacted banning private ownership. Murder rate rose after that ban.
Jamaica - 36.1 murder rate. Firearms and ammunition ownership regulated since 1967. Requires licenses. Less than 3% of population own guns.
Belize - 34.4 murder rate. Very restrictive. Most calibers illegal. $500 permit per weapon. Yearly license fee to own. Firearm license required to buy ammunition. 9mm only allowed pistol caliber. No rifles allowed.
Brazil - 24.6 murder rate. Minimum age to own a gun is 25. Illegal to carry gun outside of home. Background checks. Registration of firearms. Safety training course required for firearm license. Government keeps record of all owners. Limited quantity of ammunition allowed in possession.
Costa Rica - 10.0 murder rate. Citizens may own guns. But to do so you have to be a permanent resident, fill out a written application, show documentation how weapon was acquired, show identification documents, present weapon to Dept. of Arms and Explosives for inspection, fingerprints taken, present a psychological exam certification (after you hire a psychologist to administer said exam of course), and a certification from Costa Rican criminal archives division showing no criminal record. In other words, legal, but they make it a pain in the ass to get one.
British Virgin Islands (UK) - 8.4 murder rate. Technically gun ownership allowed with permit by law. Reality is, haha, no. Total number or registered firearms there is 48 (2 rifles, 28 shotguns, and 18 handguns). It's the UK.
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Just gonna cut to the chase here... If you look at total murder rate by country, the United States is the 112th one on the list. Clear down at a murder rate of 3.8. This is from the country where gun ownership is defined as a fundamental right by law and the world thinks everyone owns one. Which is also another fallacy by the way.
In fact according to a study by
two Harvard professors, gun policies in Europe show evidence that counters the correlation between gun ownership and violence. Several countries there that maintain high rates of gun ownership possess lower murder rates than other developed nations in which gun ownership is more restricted. Banning guns will not solve the problem of violence or murder.
"Murder rates are determined by socio-economic and cultural factors," not availability of guns.