This paper focuses on the scaling study of a capacitor-driven railgun. Railgun is an important kind of electromagnetic launcher. Although the basic principle is relatively simple, due to complicated technologies and extreme conditions, railgun is still far away from practical application. Scaling method is widely used in different field projects, including railgun systems. Subscale-system's test results and experiences can be extremely valuable for the actual system. This paper derives the circuit-scaling relationships based on circuit equations and momentum equations in a capacitor-driven railgun system. Four circuit linear-scaling methods (LSMs) are introduced, and a further approximate LSM is discussed. Finally, numerical verifications of the three models were given, including an original, a full LSM model, and an approximate full LSM model. It is shown that the performances of the full LSM model can be extrapolated perfectly to the original. The results of the full approximate LSM model can get almost the same result as the full LSM model, although some circuit equations are violated. All of the analyses demonstrate the flexibility and exactness of the circuit-scaling method. A detailed derivation and numerical validation are presented in this paper. © 2010 IEEE.