By having a gun on board, one can conduct military kinds of experiments. Exploring armoring approaches for spacecraft, warning systems for the personnel, orbital ballistic studies for the multi-body attacker-projectile-target systems. The actual presence of weaponry in space has an immediate impact on the design of certain classes of spacecrafts.
The Zvezda module of the Russian ISS sector is the direct descendant of Almaz (Salyut) space stations. The Almaz stations had the Nudelman-Rikhter gun onboard. Is an NR gun installed on Zvezda?
If yes, have any experiments been conducted with it?