It is a good hack, but it also prevents you from easily doing kiosk bag check-and-drop stuff, so there is a lot of waiting in line. And I wouldn't recommend it for international travel.
I haven't kept good track, but I'm sure I've flown over 200,000 miles in the last decade, with all sorts of miscellaneous high-value gear. The only domestic baggage incident I've had was a Canon EOS5D and 28-300mm L lens go missing on a flight from SFO->BOS. Granted that was an ~$5,000 loss, I probably should have kept the gear in carry-on. I don't think I would have wanted to deal with the process of declaring a weapon in my bags on every flight where I had something work stealing just to prevent the 1 incident out of 1,000 where there was actual threat, that's why I have insurance.
Even in the case of checked luggage with a firearm, it is usually required (and a good idea in any case) that the weapon be unloaded and disabled (gun lock, etc.). My personal opinion is that the airline is not going to call the local authorities and tell them they just turned over a bag with a weapon to you. So, as long as you don't call attention to yourself, the likelihood of getting arrested is too small to worry about.
And yes, there is a "provision" for carrying a weapon through those cities, the Firearms Owners Protection Act (FOPA) of 1986, but it only provides a defense once you've been charged. You would spend weeks or months with your life in turmoil, perhaps locked up (you're obviously a flight risk, no ties to the community) and spending significant money for competent counsel to get the charges eventually dismissed.
And this does frequently happen (arrests at O'Hare, La Guardia (sp?), JFK and I think Newark), enough so that I don't see the ruse as being worth the risk.
It obviously doesn't stop an abusive law enforcement agency from ignoring the law, e.g. the New Jersey State Police were notorious for doing that until they got slapped down enough times.
The FOPA provides a defense; legally, as a matter of procedure, all it does it allow you to get a case dismissed.
And as noted, it has specifically has not stopped travelers in the cited airports from getting arrested when they were forced to take possession of their luggage en route due to a diversion or whatever. (In fact, if you were to take gun to a hotel in such a situation the FOPA wouldn't protect you at all, would it?)
So this is going to have to a matter of trust: trust my memory and distrust notoriously anti-gun cities/states. Or not, it's your time, money and freedom on the line.
I should also note that the usual point of arrest is after the victim accepts his checked baggage, when he later declares the firearm at the baggage counter when restarting his journey.
If you buy the camoflage pattern one it is generally treated as a gun by the airport - even if you don't declare it.
It means a trip to the luggage office at the destination to recover it from the locked secure baggage area but thats a small price to pay for a free armed guard.
The main problem in the US is that baggage claim is on the street side of the airport and security is limited to a mall cop who might check your ticket tags if he can be bothered. A nice shiny camera case going around the conveyor 10ft from the open door is the main risk.
For my personal item, I use a backpack. I have a nice, big L.L. Bean backpack that fits a ton of stuff in it.
For my carry-on, I use a soft bag of the appropriate size. You might be able to compress more stuff into the hard case rollaways, but the nice thing about the soft bag is that it always makes it into the overhead compartment (unlike many of the hard case suitcases I've seen.)
But I guess if you're traveling with a lot of camera equipment you wouldn't have that option.
I think the restriction is that you're only allowed to have a personal item on international flights. Domestic flights are the same as always.
270 Euro maximum fine and an automatic charge for possession.
Better warn him not to do that again, next time he might not be so lucky.