what mango said.
my biggest improvement in using a shotgun came when i dumped a bag of .12, .20, .23, and .25 gram bb's in a bag together and shook it up and use that for my shotgun bag. you get different ranges and spreads for every shot, makes it very effective. also, they are like real firearms, the shotguns tend to do significantly better in CQB as opposed to field play and most shoot slow enough that you can have really close engagement without having to worry about hurting anyone.
they work wonders as a backup