The common Olin flare guns sold at marine supply (and WalMart
)
have plastic frames and plastic barrels: they will accept a minishell
(under 2") but PLASTIC BARRELS do not have the strength for a
shotgun shell.
12 gauge marine flares should not be fired in shotguns either.
Although I have gotten away with firing them in 18.5" no-choke
cylinder bore shotguns, if one stiks (and they will stick in a choked
barrel) it does not do the barrel much good.
As far as using a flare gun for SD, I have taken fired flare shells,
capped them and inserted a plastic shotgun wad backwards like a
badminton birdie. They will persuade stray dogs to leave your
trash cans alone without permament damage to the dogs.
Since you are responsible for collateral damage in a SD situation,
and the potential of a flare as a fire starter is high, I do not
recommend flares for general self defense use. You could end
up owing an innocent bystander a new house or car.