The definition:
"is constructed so that it cannot readily be broken open or into"
is ambiguous and that open to interpretation that it allows the use of any of the tin boxes marketed by CT, Cosco, and many others as "Gun Storage" units. If it is identified and marketed for the purpose of firearm storage, you are good to go. That said, any time the police come to your home on a firearms related matter, you can expect "unsafe storage" to be among the catch-all charges they will routinely level. Just as routinely, the charge fails in court when challenged.