Depends:
Some departments still use a 9mm for a sidearm, I dont know why as a cracked out feaned is not going to even flinch from a non fatal shot from a 9mm
Most departments standardized on the .40/.45 caliber sidearm round of choice
For rifels they either use the .223 or the standard 5.56 round…
As far as issued sidearms, most departments at least around here just get whatever gun manufactuer bids the lowest… although they are not going to give their officers some POS unreliable gun
the Glock is a very popular gun in law enforcement for its light weight, simple design and its reliability. It takes a lot of abuse/not taking care of the gun, not cleanining it for long periods of time to get the gun to jam.