It seems the interface between the locking bolt and lug surfaces are not as tight as they should be thusly holding the barrel flats down tighter to the frame table.
It may well be the bolt that needs attention, but note that the barrel flats should NOT be snug down against the action flat. There should be almost no daylight visible at the very front of the juncture, but an increasing amount visible towards the rear. This is by design. The "tightness" should be at the hook/pin relationship and the barrel breech/standing breech relationship. The barrels' breech faces should "seat" snugly and with much contact with the action breech, thus stopping the breech end of the barrel flats from contacting the action flats.
Of all the break action shotguns that I've had, the breech face has never been a component of the lock-up. And I have to admit that watching the best Perazzi 'smith in the US fitting a number of barrels for me revealed only his interest in the proper fitting of the flats and the bolt. Ejectors may rub on the standing breech but not the barrels proper.
IIRC you have a Perazzi that is mondo difficult to open. Lucio could fix that badly fitted barrel for you in a couple minutes. You'd be amazed at how easy a Perazzi is to use when it is right.
Just a thot
Of these two vastly different opinions, I'd have to agree with Stan rather than Dr. Wanker. Loss of contact of the barrels with the breech face is a problem. In fact, there is a commonly used term for it... "off face", or as the British say, "off the face". And the rear of the barrel flats should not contact the water table. All of the informed sources I've read on this subject also agree with Stan, including "Shotgun Technicana". Here's a link to a pretty good article on the subject by gunsmith Delbert Whitman:
https://shootingsportsman.com/re-jointing-a-gun/On most, but not all double shotguns, the top lever going left of center is a clear sign that the bolting surfaces are worn. This is also covered in the link above.
Of course, Dr. Wanker also seems to think the gun in question is a Perazzi, when the OP clearly states that it is a "nice Japanese box lock/extractor gun". Who knew that Perazzi's were Japanese??? Another Mystery of the Cosmos Unlocked, I guess! Same intellect that says Biden is better than Trump.
