Rifled barrels which have been machined to tight tolerances are non-trivial to make. But a smoothbore barrel is just a tube. For plenty of low-power cartridges,

Rifled barrels which have been machined to tight tolerances are non-trivial to make. But a smoothbore barrel is just a tube. For plenty of low-power cartridges, it doesn't even need to be metal. http://en.wikipedia.org/wiki/Improvised_firearm http://en.wikipedia.org/wiki/Improvised_firearm