Flare guns are used to fire flares, to call for help at sea or other dangerous things.
There are several companies that make flare guns, such as 'Hebel'. Alternatively, one might try the brands 'Wolseley', 'Webley' or 'Berridge' for flare guns.
Most flare guns are single shot.
Nope
You might think a flare would not burn in a vacuum, however, they will. The oxygen for the flare to burn is already chemically in the flare. And flare guns HAVE gone into space- as emergency equipment.
Since flare guns are used in an emergency, the possesion of a flare gun is legal by a felon if used properly.
In general, no. A flare gun, properly called a flare projector, is capable of launching only a light weight, low velocity projectile, and is not suitable for use as a weapon. If an attempt were to be made to fire a standard firearm cartridge from a flare projector, it will likely destroy the projector, with a strong possibility of injury to the shooter.
Flare guns are used in case of emergencies in many instances. If you get lost in the woods you can shoot off a flare gun to help search groups find you.
The guns are generally rated on caiber or guage. The flares may be rated on range, type, brightness, color, etc.
Flame
Not only legal, but in some cases, required. But there are places you cannot take them (for safety reasons)
S&W did fill a Navy order for signal guns in the very early 1900's. They have also made gas grenade launchers, etc.