Shotgun flasks most often have adjustable spouts, of large diameter. Unlike today, the old timers quite often loaded straight from the flask without a powder measure. The adjustable spouts on these flasks allowed for them to not only be used for powder, but a second flask could be used for shot as well. Their thin copper, or brass, bodies, with soft soldered seams gave little resistance if the powder ignited, and produced little or no shrapnel to worry about.
This example is in remarkably good condition. They are often quite battered from years in the field, or long neglect.
Hungry Horse