This work aims at characterizing the flow in the outlet of three gas injectors used in atmospheric burners and developing correlations for the discharge coefficient, air entrainment, momentum, and energy flow rates. These devices have millimeter-sized orifices, a cup-like region at the injector outlet, and the flow occurs in the transition from the laminar to the fully turbulent regimes. The pressure drop was measured and correlated as a function of the orifice Reynolds number for the three injectors. The correlations are able to predict the discharge coefficient within ±5% deviation from the measurements in the range . The axial velocity and turbulent intensity were measured at the outlet of the injectors using a hot-wire anemometer at the orifice Reynolds number of 3060, which is typical of the applications. The measurements were compared to computational fluid dynamics (CFD) solutions using the Reynolds-averaged Navier–Stokes transition model in the star-ccm+ commercial package. The results indicate the strong influence of the shape of the outlet cup-like region of the injectors on the development of an internal mixing layer and the external mixing layer in the free jet. The momentum and energy flow rates for the injector model with the largest cup are reduced to 50% and 21%, respectively, of the simplest gas injector. However, the gas jet in this injector carries 28% of the stoichiometric air before leaving the cup. These aspects must be taken into account in the preliminary design of atmospheric burners.