Theres no such thing as illegally making a rifle a pistol. When you put a brace on a LONG barreled rifle, it becomes a pistol with a long barrel. If you put a brace and a short barrel on a rifle it becomes an SBR. Yes, even though it has a brace, its still an sbr. Because SBR is defined as:
a weapon made from a rifle if such weapon as modified has an overall
length of less than 26 inches or a barrel or barrels of less than 16 inches in length; [...]
If your gun is registered as an SBR, you can legally go between rifle and "pistol" because both configurations are legally considered SBRs.
This technically means that you cant cross state lines with an SBR that wasn't originally a pistol. Do it anyway. Even if they wanted to, the ATF wont convict you for it. It would be nearly impossible to detect. With the handbook saying its ok, it would be hard to convict.
If you add a long barrel to your SBR, you can legally cross state lines because your rifle is no longer an SBR. then once you're home, reconfigure it as an SBR.
Mbogo, your second point is incorrect. A rifle can never become a pistol with a barrel less than 16" unless it was originally a pistol. If you configure a rifle as a pistol, its legally an SBR unless it was originally a pistol. Also, if it was originally a pistol, you do not need to remove it from the registry to configure it as a pistol.
What a legal clusterfuck