49 CFR 1540.111is pretty clear on the requirements for the lock. No airline or TSS agent has the authority to change these requirements.
https://www.law.cornell.edu/cfr/text/49/1540.111
(c)In checked baggage. 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:
(1) Any loaded firearm(s).
(2) Any unloaded firearm(s) unless -
(i) The passenger declares to the aircraft operator, either orally or in writing, before checking the baggage, that the passenger has a firearm in his or her bag and that it is unloaded;
(ii) The firearm is unloaded;
(iii) The firearm is carried in a hard-sided container; and
(iv) The container in which it is carried is locked, and only the passenger retains the key or combination.
(3) Any unauthorized explosive or incendiary.
https://www.law.cornell.edu/cfr/text/49/1540.111
(c)In checked baggage. 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:
(1) Any loaded firearm(s).
(2) Any unloaded firearm(s) unless -
(i) The passenger declares to the aircraft operator, either orally or in writing, before checking the baggage, that the passenger has a firearm in his or her bag and that it is unloaded;
(ii) The firearm is unloaded;
(iii) The firearm is carried in a hard-sided container; and
(iv) The container in which it is carried is locked, and only the passenger retains the key or combination.
(3) Any unauthorized explosive or incendiary.
