Some voices have said that the common practice of hiding the locked gun case within a larger and non-descript suitcase offers less security than just checking the locked gun case all by itself. The concern is that a thief can access your (insecure) suitcase and remove your (secure) gun case from it, but, apparently, due to the way that luggage is tracked, it is difficult for a thief to take an entire piece of luggage.
If you buy into this line of thinking, you might choose a larger case, perhaps even the largest one that still meets (your preferred airline's) criteria for checked luggage.