Automatic gas-checks were designed to be inserted into the barrel separately from, and before, the projectile, so that the gas-check would become located on the base of the projectile, but not attached to it, during ramming.
Fig 3 shows the base of a 10-inch studless common shell Mk I with automatic gas-check in place but not attached i.e. as it would be in the gun after ramming but before firing.