Clearly the answer is to ban all guns then, because there's no possible way that a criminal can obtain something that has been made illegal. Worked out real well for Prohibition and The War On Drugs(TM).
"Nothing else can be done. Says only country where this frequently happens."
I'll take my chances in the US over pretty much anywhere in Central and South America these days.
It's interesting that your first thought is to compare USA to developing countries.
I was more thinking about Canada, UK, Australia, etc. Other first world countries. In Australia the same gun that was used in the Sandy Hook shooting costs $33 000 on the black market. While it doesn't stop everyone from carrying out these deeds, it certainly deters many.
In the 10 years prior to the gun reform in Australia, they had 10 massacres. (— defined as a shooting involving five or more victims —) In the 20 years that followed they had one. In 2014 there was a familicide that resulted in 5 deaths. Though familicide by nature is very hard to regulate, as you can kill trusting family members with a pillow in their sleep.
Also for the record, I don't think you could just outright ban guns in the US. You'd have to start with heavy gun reform, and if they ever want to get to banning guns it would take a very long time. But at this point I just want some sensible gun reform. Mandatory safety training for starters, and universal background checks.