Best Fuel for Potato Gun
How much butane is typically in deodorant (percentage)


Here's a link to an MSDS for a deodorant; it claims the butane range is 30-60% (they don't state if this is by volume or weight). It might be cheaper to just go to a hardware store and buy a small butane cylinder, like the ones used for outdoor cooking, then you know exactly how much butane you are using.


  For a potato gun, is it wiser to use butane or propane?


My first though was butane because it gives off a bit more heat per mole (or per unit volume) when it burns, than does propane.

$$\ce{C3H8 + 5 O2 -&gt; 3 CO2 + 4 H2O + \pu{2219 kJ/mol}}$$
$$\ce{C4H10 + $\mathrm{6.5}$~ O2 -&gt; 4 CO2 + 5 H2O + \pu{2878 kJ/mol}}$$

However, according to "Common Fuels for Combustion Spud Guns", 


  The key value for comparing two fuels based on their heats of combustion is not the actual heats of combustion. Instead, the "Heat per mole Oxygen" should be used since the amount of energy in the combustion chamber is limited by the amount of oxygen present in the chamber. Fuel is added to match that amount of oxygen. As you can see from the table, there is relatively little difference between the various fuels based on their "Heat per mole Oxygen" values.


Both propane and butane give off $\pu{105 kcal/mol}$ of oxygen, so they should perform equivalently in a spud gun. The article goes on to note that, of common fuels, only hydrogen ($\pu{119 kcal/mol}$ of oxygen) and acetylene ($\pu{120 kcal/mol}$ of oxygen) produce significantly more energy under these conditions than other fuels.

Now going back to your first question, if you know the volume of your chamber, you can back-calculate the correct amount of air (oxygen) and butane.