The SpaceX Raptor engine, which applies full flow staged combustion technology, has garnered significant attention for achieving high efficiency. This study analyzes the effects of oxidizer injection methods and recess length on combustion efficiency and heat transfer characteristics in a single injector thrust chamber using a propellant combination of gaseous oxygen and gaseous methane. To this end, shear coaxial and swirl coaxial injectors were designed and fabricated, and hot-firing tests were conducted under various mixture ratio conditions. The results showed that the swirl coaxial injector generated higher heat flux compared to the shear coaxial injector, with the increase in heat flux being more pronounced when a recess length was applied. Although combustion efficiency was not significantly affected by the injector configuration, the overall combustion efficiency reached its maximum value at a mixture ratio of 3.4. This study provides important foundational data for understanding the impact of injector design on combustion efficiency and heat transfer characteristics when injecting gas-gas propellants.