There are 4 weapons in Post Void:
1. Pistol (Your starting weapon)
2. Uzi
3. Shotgun
4. Knife
The uzi, shotgun and knife are possible upgrades that can be selected after completing a level. If you're having trouble finding them in the upgrade rotation, try buying some of the permanent upgrades (ricochet bullets, enemies explode on death, compass that points to exit) so they can be removed from the rotation and allow you a greater chance at getting a weapon upgrade.