I have flown with airsoft replicas. The first time I didn't declare it, and I got called from the gate. The guy was upset about the green gas, but he knew it was an airsoft pistol and he told me that gas wasn't allowed in the baggage.
Ever since then, I've flown 5 or so times with airsoft guns. A few times with just a pistol, and once with multiple AEGs. I've told them about it, and they say it's fine. I was told that I don't have to declare it, and I don't have to lock it. Twice I have had a pistol in no case at all, but completely unloaded. Once I had rifles in a rifle-bag and I locked it after they inspected it. (More for theft than protection).
Regardless, you shouldn't have to register it as a firearm. Just telling them you've got an airsoft pistol so they know what to expect is good for Domestic.
I have no international experience with replicas.