The design, construction, and the testbed results for a novel compact gas gun injector for solid diagnostic pellets of different sizes and materials is reported here. The injector was optimized for the diagnostic requirements of the ASDEX Upgrade tokamak, yielding the possibility of a widely varying deposition profile of ablated material inside the plasma. This allows variation of the pellet velocity and the total number of injected atoms. The use of spherical carbon pellets and different propellant gases (He,N2,H2) results in an accessible velocity range from about 150 m/s to more than 600 m/s and pellet masses from 2×1018to 1020atoms. Both the scattering angle (∼1°) and the maximum propellant gas throughput to the tokamak (less than 1016gas particles) were found to be sufficiently low. The injector provided both high efficiency (≥85%) and high reliability during the whole testbed operation period and also during the first injection experiments performed on ASDEX Upgrade. The pellet velocities achieved for different propellant gas pressures and pellet diameters were analyzed. It was discovered that, although the pellet diameters range from 0.45 to 0.85 times the barrel diameter, the pellet acceleration is mostly caused by gas drag. Pellet velocities in excess of those calculated on the basis of the gas drag model were observed. Additional acceleration that increases with the pellet diameter contrary to the gas drag model may be explained by the influence of the pellet on the gas dynamics in the barrel.