While these may be to late to change, I want to mention this anyway. The shot pattern on a shotgun is about the size of a large dinner plate. A 30-foot cone is going to scatter the pellets so wide as to not seriously injure a target (making it ineffectual as a hunting weapon). A better way would have been to allow pellets to reroll the miss chance from concealment.
The blunderbuss was not really designed to fire a slug. It should really only fire pellets. It aslo had about a 5' shot pattern at 60 feet, so using a cone-shaped attack works for it. The same could be said for a dragon pistol or a sawed-off shotgun, since they would have a wider shot pattern because the smaller barrel length.
I think this is one of those situations where the game needs an effect that real life doesn't have. And that's ok. Nobody questions the complications of using a fireball in an enclosed space with a limited air supply.
Sometimes art need not imitate life.
A few of the DM's that I have run with have and it was not a pretty sight
I don't mind the cone effect for the blunderbuss or the dragon pistol, but the shotgun is more advanced. I also like the idea of a reroll against concealment with shot.
Now that I think about it, I will probably houserule the shotgun's cone to a line. It gives the weapon more control (as an advanced firearm), while still keeping a similar rule to the blunderbuss and dragon pistol. This also prevents a single shotgun attack from hitting a line of 8 creatures at 20 feet (see the 30 foot cone example on Page 215 of the Core Rulebook). Talk about a roomsweeper.