That one I'm not sure I blame them for. As I said the "machine gun ban" was a last minute floor amendment, and no one really knew what it meant, although they did know the intent of it's author. I suspect the NRA thought, if they knew at all, that because of the poor wording, the ban would be unenforceable. OTOH, I don't recall it being challenge in court either.
Machine guns and NFA weapons in general have been so demonized that virtually no one will challenge the First Federal Gun Gun Control law. Regarding that law, the sponsors wanted to treat handguns the same as Machine Guns. In the late 1930s, Roosevelt's attorney general had asked Congress to require every owner of a rifle, shotgun, revolver or pistol to register his weapon with the bureau of internal revenue. A tax of one dollar would have been payable for every firearm sold. The attorney general stated that this legislation would broaden the scope of the NFA and "would place a potent weapon against criminals in the hands of law-enforcement officers." (See Chi. Daily News, May 4, 1937, at 4.)
It took until the very '94 Brady Act that this very Gun Bill is "improving" before the federal government got in the position to be able to register, each and every firearm sale from a dealer.
If Canckles becomes President, and the 'Rats retain control of Congress, BOHICA, big time.