Purchasing a rifle or shotgun only requires that you be at least 18 years of age. A photo ID is required, and the instant background check is required to be completed. The background check is run by phone at the store when a purchase is completed, and normally takes only a few minutes. If you have the required handgun purchase documents the background check for long gun purchases is not required.
photo credit: http://en.wikipedia.org/wiki/File:SEAL_IOWA.png