Pre mix guns consist of a series of tanks of ready-to-serve beverages, connected via hoses to a single nozzle used by the bartender. post mix guns also consist of a single nozzle, but they are connected to individual tanks of beverage syrup and carbon dioxide. They get their water from a dedicated water line.
Many large soda producers (for example Coca-Cola and Pepsi) produce their own pre mix. Typically, in addition to their cola, they will combine on the gun their lemonade, a tonic water and their diet cola. For example, in the UK, a typical Coca-Cola premix gun will contain Coke, Diet Coke, Schweppes Lemonade, and Schweppes tonic. A typical Pepsi gun will contain Pepsi, Diet Pepsi, Orange Quench, R whites lemonade and proprietary tonic. However, in restaurants and fast food outlets, the ancillary soft drinks will be replaced with named soft drinks (rather than mixers as in bars). So Sprite would substitute Schweppes lemonade; 7up would substitute R whites in Pepsi machines. Additionally it would be possible to have other drinks (not often used in bars) like Fanta, Ice tea, root beer etc.
Some recent innovations of soda guns has included the provision of orange juice, cranberry juice and even an energy drink.
The advantages of dispensing from guns instead of bottles include lower cost of transport (because water is added at the point of sale), convenience (no need to locate and open bottles or cans; a single canister contains much more product than a bottle), a freshly carbonated drink for the consumer, and a much more profitable product for the venue (a glass of post mix costs about 1/40th the price of the same amount of drink from a bottle).
Post mix guns have the advantage over pre mix guns that each drink can be mixed more precisely, with custom settings for the water and carbon dioxide mixture. Pre mix guns, however, take up less physical space.