If you can hit a person square in the chest with buckshot from 25 feet away without even aiming down the sights in real life, you should be able to do the same in a game with "futuristic weapons."
Also, since every shotgun in this game is pump-action, it stands to reason they should all have the same rate of fire. Of course for variety they could have different stability and damage. The ones with more damage would have longer barrels, and the ones with shorter barrels would always have a lower ammo cap 'cause the mag tube couldn't be longer than the barrel.
Any other ideas?
EDIT: I forgot about the Keltec KSG. It has dual mag tubes and is pretty compact, so short barrels wouldn't necessarily take a hit on ammo cap.
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Play BF for Shotguns that work. Nothing better than trolling Dom games with a DBV12 and frag rounds in search of hate mail.