For this one you need NFA paperwork and tax
But not for this one:
Why? Because the bottom one is a “brace” and the top one is a “carbine conversion”. There are several visual and physical differences, definitely enough for it to be a different system. Let me give you the link to CAA so you can see the ludicrous legalities. The NFA34 just needs to be deleted. I’m thinking that’s one of President (elect) Trump’s plans, and why he has a 2A Coalition working on this stupidity. They need to throw in a 10A subdivision with it though. The 2A was there before the 10A, and 10A says “if it’s not in here it’s up to you, if it is in here it’s the law you abide by”.
Here, have another link to the CAA site about this.