According to official FBI regulations, agents may only carry bureau-issued or approved handguns. Once FBI agents pass training, they can be issued a Glock Model 22 or a Glock Model 23.
Both of these types of guns are .40 S&W and have a magazine capacity of 15 rounds. They are the newer versions of the Glock 17 and Glock 19, respectively. Different types of agents, such as those on the Hostage Rescue Team, may be issued different weapons to fit the requirements of their duties. According to the FBI website, agents may also carry personal weapons with them, as long as they are on the approved list.