K17

The K17 has been seen in the Ultimate Edition screenshots.

Rockstar has not shared more about the K17 yet. This page grows as new trailers and screenshots come out.

More handguns

Capo pistol
Seen in official screenshots.
ES9
Seen in the Ultimate Edition screenshots.
Morgan
Ultimate Edition
Seen in the Ultimate Edition screenshots.
Mustang .357
Seen in official screenshots.
Nipper .38
Seen in official artwork.
Pistol
Seen in the first trailer. Earlier in games including GTA IV, GTA Chinatown Wars and GTA V.