You can cross state lines IF you let the ATF and state police (I believe) know. I don’t know all the rules surrounding SBRs, as I don’t have one. Suffice it to say that regulations in regards to SBRs are much more stringent as well as vague than they are with pistols.
If you have an AR pistol, make sure you have an ATF approved brace on it. And make sure you don’t have a vertical fore grip. As of right now, that’s pretty much all you have to worry about and you can treat the weapon like any other handgun. Register it as an SBR and you open up a whole new can of worms and you better make sure you are aware of all federal and state laws regarding NFA classified firearms.
People can do what they want. But, I’ll stick with the pistol, unless I’m forced to reclassify.