We present frequency-domain Kirchhoff migration, which is a fast and high-resolution imaging algorithm for ultra-wideband radar. The algorithm is developed by combining three conventional methods: Kirchhoff migration, frequency-wavenumber (F-K) migration, and inverse boundary scattering transform (IBST). Frequency-domain Kirchhoff migration is compared with conventional methods in terms of the computational load and resolution by applying the methods to measurements made for a handgun, which is an important target in various security-related applications including airport security.