Here is a set of screenshots with explanations written: https://imgur.com/a/pVh4FxR
The short version: The mp-133 510mm barrel has too low of an accuracy, the same as a sawn-off 350mm remington. While a 508mm remington has an accuracy of more than 3 times that. ( The screenshots have extensive explanations on each barrel).
Plus most shotguns are a bit too inaccurate across the board. Also the shotguns with low accuracy tend to spread so the pellets completely avoid the center of aim, which I think is a bug too? Because on the longer barrels the shot patterns is realistically unpredictable.
got it. we need to figure this out cause accuracy of shotgun barrels in not used to actually spread the pellets. there is other parameter for pellet spreading