This is
49 CFR 1540.111.
This is the pertinent part of that CFR. I bolded the exact line that's important:
A passenger may not transport or offer for transport in checked baggage or in baggage carried in an inaccessible cargo hold under §1562.23 of this chapter:
...
(2) Any unloaded firearm(s) unless—
...
(iv) The container in which it is carried is locked, and only the passenger retains the key or combination.
When it comes to
TSA-Approved locks, the passenger is not the only one who "retains a key" to open it, as outlined in 49 CFR 1540.111.
Obviously. That's the whole
point of the TSA-Approved lock in the first place. The end result, however, is that TSA-Approved locks will not be acceptable for securing the hard-sided locked cases containing unloaded firearms in checked baggage.
EDIT: After thinking about it some more, I don't think it would apply since it wasn't a TSA thing in the first place. Henceforth, I have removed it.
Irony, meet fail. Fail, meet irony.