That we require mufflers on our cars yet forbid them on our guns.
And don’t throw out that you can get one in some states if you pay the tax and such. The intent of the NFA was back door gun control by making the tax so high no one would be willing to pay it. In 1934 a Thompson SMG sold for less than $200.
Thanks to the miracle of inflation, $200 isn’t what it used to be and now it’s not completely insane to pay a $200 on a $15K gun. (The Hughes amendment is a whole ‘nother rant)
But it’s still crazy to pay a $200 tax on a $20 muffler.