Your first problem is thinking they've been working on this for 7 years lol they did do a little thing called 'Skyrim' in between. Secondly, most people would agree this is their most stable game at launch so far. If you're comparing it to 3/NV after a year of patches and fixes, sure. But that doesn't count. We're 3 weeks in. Lastly, as the other poster mentioned, you might be referring to the ejection port where spent casing are ejected from the breach...
Are you talking about one of the additions to the muzzle of the gun? Compensator or one of the others? You said square thing at the end of the barrel and that's all it could be. In any case this isn't a 'bug'. Poor/lazy design maybe but not a bug. But I'm just guessing at what you're talking about since you can't paint a clearer picture. Maybe a screenshot?
Why do people on forums always feel like they need to get mad when someone corrects their wrong statements. And I posted that before I saw you replied with "the little square piece that they put on the end of the shotgun looks like an obstruction in the barrel" You're probably talking about the compensator as I already said. It's a mod for the shotguns muzzle.