Thats how I picture it. Your right Nevada doesn't care but california cops do care. You cannot bring ammo back from another state. So I'm assuming california cop sits outside a gun store in Nevada, sees a car with CA plates and either follows or radios ahead to another cop sitting at the california line. The line isn't far, maybe 30mins. They pull you over once you cross for something frivolous and they start asking questions. Id assume they have an agreement with Nevada somehow. This could all be bullshit rumors but I can't put it past CA in doing something like this. Probably has more to due with gang members who can't pass a background check here so they drive a couple hours.