To fully upgrade all weapons, you need to access 12 Power to the People stations around Rapture, including the one in Cohen's room (May be unattainable if you kill Cohen after you finish his masterpiece, please comment if you can confirm either way).
Locations are:
- Neptunes Bounty (Freezer bottom floor)
- Arcadia (Tree Farm)
- Farmers Market (Worley Winery, bottom floor)
- Fort Frolic (Le Marquis D'Epoque)
- Fort Frolic (Sinclair Spirits)
- Hephaestus (Close to geothermal control room, on the ring where you enter it)
- Hephaestus (Kyburz office)
- Olympus Heights (Mercury Suites, lobby on the opposite side of the elevator)
- Olympus Heights (Sander Cohen's apartment, see spoiler)
- Apollo Square (Hestia 4th floor. Access from 5th.)
- Point Prometheus (Atrium, next to the dead big daddy)
- Point Prometheus (Optimized Eugenics - Eugenics Analysis)