I don't think that plastic will ever be a reasonable material for making a firearm, no matter how good the printer gets. Otherwise we would see somebody making plastic guns using industrial injection molding methods. But all firearms on the market and even those produced in basements are pretty much completely made out of metal. Some use wood or plastic, but usually only in decorative pieces, or the stock of a rifle, and definitely not in the barrel or chamber (pardon my terminology if I'm using words incorrectly). The only really viable means to 3D print a firearm is via something like
laser sintering, and even with the patents expiring, the cost of those machines will still be in the 10's of thousands of dollars. If somebody is willing to spend $35,000 to produce guns, then 3D printing is definitely not the most efficient way of doing it. Sure prices will come down, but so will the prices of all the other methods of manufacturing firearms, which means that 3D printing will likely never be the best way to do it. 3D printing is great for one-off prototypes, but once you want something resilient that can last, and that doesn't take huge amount of resources to make, you pretty much have to move to other production methods. That being said, I can definitely see 3D printers being used for some parts of the production, but possibly just for making molds for more traditional methods to be used in the actual production.