Is there a problem with buying multiple handguns in the same day? How does reporting to ATF change anything?

If you buy more than 1 handgun within a 5-day period, you don't have to do anything different, but your FFL has to list them on a special ATF form (#3310.4 - Report of Multiple Sale or Other Disposition of Pistols and Revolvers) with your name and ID info, and fax it to the "Multiple Handgun Purchase Division" of the ATF and your state.