The FBI has access to most weaponry that the military uses, actually, but this isn't really well known among the public. Imagine if terrorists crashed into some Federal building. The FBI is going to need whatever they can use to aid the.
The Hostage Rescue Team (HRT) is the United States Federal Bureau of Investigation's most capable and best-equipped tactical and counterterrorism team The HRT is trained to rescue U.S. citizens or others who may be held illegally by a hostile force, either terrorist or criminal.
The HRT's capabilities are more advanced because its operators (assault and sniper teams) serve full-time and train daily. HRT operators are assigned to one of three teams, one of which is a designated maritime team. The three teams rotate through three 60-day cycles: training, operations, and support. During the training cycle, the team refreshes its skills and takes part in exercises. During the operations cycle, the team is available for deployment. During the support cycle, the team works on special projects and maintains the HRT’s equipment.
One of the chief capabilities that distinguishes the HRT from the FBI's SWAT teams is its ability to fast rope, a technique where the assault team descends a thick rope from the side of a helicopter. HRT also possess advanced "breaching" capabilities unlike the Field Office SWAT teams.
Some of the HRT's roles include:
High-risk raids, searches, arrests, and warrants
Manhunt and rural operations
Cold/extreme weather operations
Force protection for FBI personnel overseas
Assistance to military special missions
Additionally, the HRT has performed traditional law enforcement roles during hurricane relief operations, tactical surveys, and on occasion, pre-positions in support of special events such as the Olympic Games, presidential inaugurations, and political conventions . Another one of their main purposes, according to Special Agent Whitcomb in an interview, is to handle the FBI's overseas actions. He also hinted at Black Operations as well.
The HRT is known to train with units such as the Army's Combat Applications Group (otherwise known as 1-st SFOD-D Delta Force), France's GIGN, Britain's SAS, Australian SAS, Germany's GSG 9 and other international units.