Why an undersized shell could destroy its gun

What happens if a nine-inch shell is loaded into a ten-inch gun?

The poor fit can make the projectile yaw and jam in the bore, potentially bursting the gun and killing its crew.

Without a proper fit or effective wadding, the smaller projectile could oscillate and corkscrew rather than travel cleanly down the barrel. The extra clearance allowed it to yaw and strike the bore instead of remaining aligned with the gun's axis. In the worst case the shell could lock against the barrel while propellant gases continued to build behind it. The result was a burst gun and lethal danger to the crew. An engineering report's vocabulary for progressively worse motion gives the amusing question a concrete connection to ammunition compatibility and ordnance safety.