If you pick up the regular shotgun at your hidout (from starfish island) and then go out and find the spas or benelli shotgun out in the city, your 50 shells from the regular shotgun will be transfered to the new upgraded gun you picked up. If you were to pick up the upgraded shotgun without doing this you would only recieve 20 or so shells but this way you get roughly 70. This trick works with other guns also that share simular bullets.