The difference between hard and decorative chrome is:
Decorative chrome is applied by first copper plating the steel as a "base coat" then bright chrome is applied over that.
Look at this process as painting wood. Paint is ON the wood in layers.
The decorative chrome is in layers ON the metal, can crack, chip, and peel off, plus moisture can infiltrate BETWEEN the steel and the top layers.
This means that the steel can rust UNDER the chrome and you won't know it until the plating starts to bubble up and peel off like an old decorative chrome car bumper.
Decorative chrome, along with it's copper "primer coat" is a fairly thick coating, and since the layers allow the chrome to "give" when impacted, it would quickly flake off a shotgun bore.
Hard chrome is applied by a special process in which the chrome is applied directly to the steel, and actually "soaks" into the pores of the metal.
Look at this like a stain on wood. It's IN the wood.
Since the coating is bonded into the metal, hard chrome can't chip, crack, or peel, and since there IS no "between" the steel and the plating, moisture can't infiltrate under the chrome.
For this reason, hard chrome can't rust under the plating since there is no "under".
Hard chrome plating is exceedingly thin, and VERY tough and durable, since it uses the steel itself to support and toughen the plating.
However, the strength of hard chrome is directly related to the base metal it's applied to.
A soft metal will still be impact dented or scored, and if impacted beyond the capabilities of hard chrome it can chip.
This is why most platers won't process aluminum frames. The aluminum is simply too soft and will dent under the chrome.
For the same reason even chrome won't protect an older shotgun barrel made before steel shot, because it's made of a softer steel.
Newer guns made for use with steel have (usually) thicker, harder barrels.