It is illegal for any person, other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector to transport any destructive device, machinegun, short-barreled rifle, or short-barreled shotgun across state lines without getting permission from the ATF. 18 U.S.C. § 922(a)(4); 27 CFR 478.28. So if you are planning a trip out-of-state and want to take your short-barreled rifle, short-barreled shotgun, and/or fully automatic weapon with you, you need to get approval from the ATF first using ATF Form 5320.20.