I've had TSA make me remove TSA-approved locks from gun cases and replace with non TSA-approved locks. (I've used both Brinks and Master. My preference is for keys.)
Their reasoning, and it kind of makes sense, is that, if you read the rules on your airline's website, you are the only person allowed to have access to the firearms. No one else. TSA inspects the opened case, you lock it, you retain the key (or combo). A TSA-approved lock has a small key slot in it. There is a set of "master keys" at every TSA point that allows TSA to unlock luggage for visual inspection. Subsequently, every TSA agent has access to your guns., thus, no TSA-approved locks allowed on gun cases in checked baggage.
Now, have people flown with guns using TSA-approved locks? I'm sure they have. Does every TSA agent know all of the rules? I highly doubt it.
Also, good call on the gun case. I once found a rifle on the side of the taxiway; it had fallen off a baggage cart. Cheap wal-mart special case. Broken completely open. Protected the gun, but I'm sure someone had to duct tape it back together to continue its journey.