Well, except regarding robots they are only weaker shells than the ones we know at the moment, right ?
The one programmed in to the game currently?
As far as I know the buckshot damage roll was taken from the PC game and tossed in to the PnP in dice roll format.
The PnP book states that the slug gets a range increase.
Note that using slug ammunition in a shotgun increases the range by 5 hexes, and firing a double shot (both barrels) decreases range by 3 hexes.
The EMP of course deals pure electrical damage to robots. (This is nice and simple. But I suggest if it doesn't kill the robot outright it should roll a percentage to see if it temporarily disables the bot for a certain number (like 1 to 3) turns. This would mean powerful robots have a chance to suffer the curse of being electronic.)
And the rubber bullet shells I can only see as a tool to increase the chance to knockdown, blind, or disarm someone. (The PnP once again doesn't state what special things the rubber bullets do.)