Spud guns bring to light an impressive amount of science and mathematics. In the search for higher performance, we adjust the numbers during construction. More power, higher muzzle velocities and farther shooting distances are usually the goal. One of the most important and fundamental aspects of launcher design is the chamber to barrel ratio. The volume of the chamber is usually the larger of the two. A large chamber does not always equate to more power. Imagine a giant chamber as large as a grain silo with a barrel 3 feet long sticking out the top. The potato would exit the barrel before the combustion had reached maximum force, the chamber might explode as well. Now imagine a chamber 1 foot long with a barrel 1 mile long. Obviously, the potato would never reach the end of the barrel as it would create a vacuum behind itself.

So what is the ideal C:B (chamber to barrel) ratio? For combustion launchers, the ideal balance seems to be a chamber one and a half times the volume of the barrel. This is expressed as a 1.5:1 ratio or simply 1.5. The actual performance of many different C:B ratios has been measured to help you make the right decision. This information is available at the C:B test page. So how is volume calculated? Using math, you can measure the length of the chamber or barrel and apply the formula, (radius*radius)*pi*length=volume. This works well but does not account for irregularities in chamber construction. A more accurate way to measure the volume is with water.

Simply fill the chamber and measure its volume. It helps to have a couple accurate graduates whether homemade or store bought. For instance: The volume of an 11 inch chamber might measure 2600ml. In order to have a 1.5:1 ratio, the barrel needs to measure 1734 ml by volume. A 1.5 inch diameter PVC barrel holds 31.9 ml per inch so a 1734ml barrel would be 54.4 inches long. A 2 inch diameter PVC barrel holds 53.9 ml per inch making the 1734ml barrel shrink1734ml barrel shrink to 32.17 inches long. To make things even easier, use a spreadsheet program like Excel to create a C:B ratio simulator...or just download the file below.

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