On firing , the intense powder gas pressure flattened the disc against the base of the projectile , increasing the disc 's overall diameter .
On firing , the intense powder gas pressure forcing the collar against and into the rifling , forming a gas seal .
On firing , the intense powder gas pressure flattened the disc against the base of the projectile , increasing the disc 's overall diameter , and forcing the collar against and into the rifling , forming a gas seal .