Before it got to cartel
Well a Belt Fed 50 converted to semi auto is going to set you back $8000-$10,500, I'd think it would be easier to steal a fully automatic one from some army in South America. It really isn't something that most gunshops stock.
Of course the Billons in profits the dealers make, 10 grand is peanuts.
As long as these guys are reaping these kind of rewards they will find weapons, and even if ATF gets tracking on long gun sales near the border, they'll just start buying farther north.
The drugs are the problem, and unless we as a country want to really declare war on drugs it just going to continue.
Gun control in America will not solve the problem, or save a single life here or there