This is a valid argument only if owners plan to defend themselves within a blast furnace. All this really is, is a way of pretending not to be focusing on inexpensive metals. Police and the military are invariably exempted from such laws. If the true aim is to disarm the poor, it makes sense that the police and the military should be exempted from the law. By their very nature, laws to stop inexpensive firearms cannot stop such firearms from reaching criminals.
Inexpensive arms are inexpensive because they are easy and cheap to manufacture.
They can be manufactured in a criminal's basement. It isn't difficult. Firearms are just pieces of metal, stamped or molded. It doesn't take a rocket scientist. But if it takes a criminal, then only criminals will have these arms.