1. They can check your weapon to make sure you are transporting lawfully.
2. I don't see that this includes anything to do with magazines that are not inserted in your firearms, whether they have bullets in them or not, or even how many they can hold.
Even if they find hi caps, so what? Burden of proof is on them. The fact he just let you walk away like that is odd though.
When I transport my AR it's in a case that has dual compartments, one for the rifle, and one for other stuff, including mags, so all I would open is the rifle part. I would not consent to a search of any other compartments or my other personal effects. I keep an empty 10 round (not blocked but a real 10-round) magazine in my AR with the bolt locked back so it's real easy for them to see it's not only unloaded but they also need a tool to remove the magazine.
If you want a picture of the future, imagine a boot stamping on a human face — forever.
Government Official Lies
. F r e e d o m . D i e s .