Well, Iu2019m no expert, but I have crossed into and out of Mexico many times. The border guards will typically ask you questions to help you avoid committing a crime. One such question would be do you have anything to declare.But if the guard didnu2019t ask, you would be wise to tell them exactly what you wrote here. Declare the weapons, tell them that you therefore canu2019t pass, and ask for instructions on how to turn around with their permission.(Iu2019m assuming that youu2019re legally allowed to have these weapons and youu2019re transporting them legally within the US. So since you havenu2019t actually crossed the border yet, thereu2019s no crime.)That should be the end of it. My guess as to how they would handle it would be that they would allow you to simply turn around. If they were very cautious they might have you pull to the inspection station, get out, let them retrieve the weapon safely, then hold the weapons while you drove back across to the American side. And then return it to you, with some kind of verbal warning to be more thoughtful, plus possibly a thank-you for being honest and cooperating so well.