Malorian Arms 40k

The STC of this weapon was found deep in the vaults of a Techo-barbarian Hive city located in western Merica. During the Unification wars a particularly fierce warlord held out against the Emperor for 40 years. His success was largely due to his prolific use of gorilla warfare and nuclear arms. When he was finally killed it was noted that he had one prosthetic arm and a heavily modified version of this weapon. The Emperor seeing little use for it sealed the STC away in his vaults for millennia. It was only until Belisarius Cawl was given access to these vaults that it would be reintroduced into the galaxy. He largely left the STC unmodified scaling it for both astra militarum and Adeptus Astartes use.

Its noted to be a modified automatic Stubber with an integrated computer and rail gun system. The computer allows for an neural link between the pistol and its user. This has several benefits: the pistol fires when you think it, you know the exact number of bullets in the magazine and chamber, you can instantly activate the rail system allowing for improved penetatation or simply use the pistol as a standard stubber which increases the potential for ricochets.

Pistol:
https://www.thingiverse.com/thing:4728071