"The cartels" is not a single organisation. Yes the largest best-organised gangs could build firearms factories - or more realistically just bribe some soldiers or cops to "lose" a few truckloads of weapons, just like they do with automatic weapons right now.

But there is a huge difference in a weapon being available by the dozens to the highest tier of the top 2-3 cartels (like machine guns or rocket launchers or IFVs are right now), and a weapon being available by the hundred thousands for anyone with a few dollars to their name!