Not sure if still the situation, but in the UK it was the requirement that the gun was packed in a locked gun case and the ammunition was packed securely, separately from the gun (I'm not certain but I think it could be in original packaging in a case). The reason being if someone other than the owner picked up the gun case and opened it, the worst they could do is beat someone with an unloaded gun. If packed with ammo too, there was a higher potential for an incident. This obviously doesn't fix this issue as we understand in this situation, where the registered owner was the person committing the act.
This was for sporting weapons in UK, not just general firearms, but were permitted for flights domestically as well as internationally, subject to license, firearm and identification checks.