A stainless Walther PPK is used by Seigen Hayami in the OP sequence of Un-Go.
SIG-Sauer P232
A SIG-Sauer P232 was used by a Security Police officer in "Murder at the Ball" before she was killed. Public Prosecutor Office agents sent to the Sasa estate in "House, Unmasked" use P232s as their main sidearms. Shinjuro and Inga criticize them for carrying firearms, which is correct since actual Public Prosecutor employees, regardless of their status, are not lawfully allowed by the Ministry of Justice to be armed. However, this may be due to the fact that in the Un-Go world, Japan had just been through a major war with several terrorist attacks that the MOJ had allowed all law enforcement personnel including Public Prosecutor employees to have pistols with them for protection in light of another terrorist attack.
Assault Rifles
AK-47
Unknown individuals are seen fighting in Japan with AK-47s in a flashback sequence in "Masked Mansion".
Machine Guns
Browning M2HB
A Browning M2HB is seen mounted on top of an Abrams MBT in "Masked Mansion" during a flashback sequence. A Public Security Force Humvee in the Greater Tokyo Area in "House, Unmasked" on patrol while conducting surveillance also has a M2HB. It's also seen in "The Phantom Statue" on top of a Type 10 MBT.
Grenades
M67
Several M67 hand grenades are seen on a table next to the AK-47-wielding gunmen in flashback sequence in "Masked Mansion".