I ordered breech seals from Hatsan awhile back, 2 packs of 4 each considering the number of Hatsans I own (10 as the Webleys use that same breech seal). They screwed up and only shipped one pack and when they fixed things and sent the 2nd pack of 4, included 11 more for the inconvience so I now have 19...lol.If you want a couple, pm me your home address and I'll stick a couple in an envelope and send them to you.
Then that's almost definitely the reason. One good slam shut is all it takes to bend a barrel. And when it's the entire cocking force pushing the barrel up, that's more than enough to cause damage.