YouTube Banning assembly videos is not "against the Second Amendment" at all. Your right to own a gun is not affected in any way by not being able to watch or post a video about assembling the gun. Worst case, you'll need to read (or write) a manual.

It's not even against the First Amendment, because that's only about the government suppressing political speech. Besides, the First Amendment only guarantees your right to speak; it doesn't guarantee that you'll have a platform to spread your speech, or that anyone will listen to you.