According to Wikepedia (which I'm sure you've already looked at), it looks like one might encounter many types of pistols in BP service up until and likely through 1994- the issue weapon was a .357 revolver, but agents/officers could purchase and carry their own alternative if they so desired. In '94, they adopted the Berretta 96D in .40 SW caliber.
http://en.wikipedia.org/wiki/United_...Patrol#Weapons