I have several new style flare guns, when the flare kit goes out of date its cheaper to buy the kit than replace the individual flares. I would never consider putting the lightest of shotgun loads in them. I have never pulled a flare apart, but i dont think they have a charge in them just a primer.
The old mil-surp flare guns I have seen would probably handle a light charge load. They have also been more expensive than many low end pistols.
As a bit of a side note. I heard of someone
trying to shoot a seagull at about 5 yards with one. Apparently they are only accurate enough to hit the sky.