One or more ram accelerator devices may be used to form one or more holes in geologic or other material. These holes may be used for drilling, tunnel boring, excavation, and so forth. The ram accelerator devices propel projectiles which are accelerated by combustion of one or more combustible gasses in a ram effect to reach velocities exceeding 500 meters per second. An endcap may be deployed within a tube of the ram accelerator device to prevent incursion of formation pressure products such as oil, water, mud, gas, and so forth into a guide tube of the ram accelerator. During operation the projectile penetrates the endcap and at least a portion thereof impact a working face. In some implementations a purge gas may be used to form a ullage between the endcap and the working face.