Sure, you can find examples to support your view, but they still make a teeny-tiny fraction of guns used in crimes. Rifles in general make up a minority of guns used in crime. If one were to look where to focus the most attention for new gun laws, it would be handguns.
Though I will have to look more into the article on the LA Gangs. I haven’t heard of anything so sophisticated. Just from the picture, those lowers look well finished and anodized, which honestly I wouldn’t expect. But then again, articles like this are notorious for getting details wrong. You can find articles showing what was found, and it doesn’t match the description in the article at all. Though I have read about some gangs using an “Armorer” who basically go around buying things legally, as their records are clean, and then equipping members of the gang.
