(sorry for late response) for what i think, you have to spin the charger (where you put the bullets, idk how is the name in english), after that there are new items in the store until you do it again.
said that i can go back to my previous message, a larger gun can make you produce money for a longer time with the combo i said, so you don't have to worry about losing a item you want from the store.
hope this helps ya in the game