You have to have a ATF Form 6. and you have to buy your liscense ahead of time in order to get it. Takes about 2 weeks if you fax in the info. You can download the form from the ATF website. I think there is a $25 fee.
About 10 of us hereabouts go deer hunting down in Wisconsin. I used to take the guns across because I'm a dual citizen and don't need the Form 6. Last few years we just borrow guns from local friends--most Americans always have a couple extra laying around.
A couple of years after 9-11 I showed up at the border with 10 guns. The American border guards had all been changed--they all looked to me to be about 15 years old. The young lady I first talked to said you have to have a form 6. I said you can't issue one to me--because I'm not a non-rersident alien--I'm a non-resident citizen. I couldn't get real snarly with them cause I got 9 more guys coming down in a few days wanting to deer hunt so I ask to talk to her boss. Her supervisor was a little older but I don't think he shaves on a regular basis yet. He told me he had called his boss and was told to treat me just like a Canadian--you have to have a form 6. I told him to call ATF and he told me he couldn't do that. I asked why and He told me he can't just go talking to another agency without approval from his supervisor. I ended up leaving the guns on the Canadian side--what a hassle that was--trying to find secure storage 130 miles from home.
I called my nephew and the next day he showed up at deer camp with 22 different rifles. My guys had a rather nice selection to choose from at the range the next day. I like buying those hunting liscenses over the counter too- sometimes they ask for ID but a drivers liscense will do.