Arguing that something a hundred thousand people still have three decades after further supply was banned seems like a dead end. I don't know that there are a 100,000 purple italian silk ties in the country, but that would not make such a thing rare. The term "common" really begs the question, "Common for what?".
That can't be the problem with the argument, since that isn't the argument.
The OP's point is that the government shouldn't be able to prohibit an item as uncommon where it is the government's act that produces the uncommon quality of the item. That reasoning doesn't hinge on the item once having been common. The reasoning he critiques still rests on a fallacy of circular reasoning, since that reasoning begs the question of why it is currently less common than it might have been otherwise.
Couldn't we say the same things about rockets launchers and hand grenades or mortars? If they weren't so expensive (because of limited supply) and restricted by the NFA wouldn't more people have them too? They cost very little to make and so could be sold cheaply and without restrictions anyone could buy them.I think the OP makes an excellent point that I had not ever considered, namely that full auto weapons are uncommon because of restrictions.
I think the problem with the argument is showing that they were ever in common use.
That can't be the problem with the argument, since that isn't the argument.
The OP's point is that the government shouldn't be able to prohibit an item as uncommon where it is the government's act that produces the uncommon quality of the item. That reasoning doesn't hinge on the item once having been common. The reasoning he critiques still rests on a fallacy of circular reasoning, since that reasoning begs the question of why it is currently less common than it might have been otherwise.