Oh well. Clicked on the button as soon as the clock turned 10 and the inventory was already gone. Either someone was a fraction of a second faster than me, or they built a bot to do it. At least I have my permit for two.
You have to click the browser's refresh button and click again as soon as it refreshes. Then as soon as the "A" shows up, you have to click the "A", then "Book Permit". It IS quite possible that people have a bot programmed. But it is also possible that they have a faster (or shorter) connection to the rec.gov site, so they would get the updated screen with the "A" displayed before others.
But there will be quite a few more slots showing up at random times on May 1, starting at 7 AM Pacific time. With the random times, everyone has a more fair chance of getting something they want.
