A multi-channels high resolution dispersive interrogator with at a high sampling rate has been developed to measure shocks pressure levels by Fiber Bragg Gratings (FBGs). Two FBG orientations are compared numerically and experimentally. The first one is along the cylindrical target axis, thus the grating spectrum is "blue shifted". The second orientation is perpendicular to the target axis and the grating spectrum is "red shifted". The interrogator uses a femtosecond laser source to cover the C+L band spectrum. The source repetition rate (100 MHz) fixes the spectra acquisition rate. The wavelengths are basically converted to time using a long telecom fiber. The time-multiplexed spectra are recorded with 400 points by a fast oscilloscope (40 GSa/s). The experimental setup is a Tin plate impact on a PMMA target performed in a 35-mm single-stage gas gun. An impact at 510 m/s generates a pressure level of 1.69 GPa during 5 µs. The performance of the dynamic interrogator and the wavelength shifts in the two FBG configurations are discussed.