http://www.atf.gov/firearms/050406_firelauncher.htm
Don’t shoot shotgun shells and they will fire at least a few rounds. There is a test complete with pic’s floating around on the ‘net but I can not lay my hands on it atm (at work) The flare guns fail, but they break apart where the front trigger guard meets the frame. The barrel stayed intact in the tests.
Not something I would want to fire personally, but considering the ease of manufacture and the unregulated nature of the flare guns themselves, it’s not an idea I will be forgetting any time soon. :-)
The ATF only tested Orion’s plastic flare guns. Presumably metal ones, such as an older all metal Orion or Olin or the currently available GECO1 might fare better shooting .38. But the inserts go for $80. As a close range self defense weapon would just the flares be adequate or is a .38 round that much better? Do the flares light off and burn as soon as they come out of the barrel? If so, would they act like willy pete? A pack of 5 flares goes for $60 so the reducer/ammo method is not that much more expensive (assuming one does not really need the gun to also act as a signaling device).