IIRC, he also used a small bottle of high-pressure gas within the projectile to decrease the decelerating effect of the flow-separation “vacuum” that develops immediately behind the projectile. Supposedly this helped the shell gain a lot more altitude.
Of course, in the case of a direct-to-orbit gun, that “high pressure gas bottle” would probably have been replaced with a rocket engine.
“Base-bleed” ammunition. I remember reading about that with Israeli and South African artillery.