If you want to build your own gun, most gun kits provide the barrel and lock parts, while requiring you to build the stock and assemble the other pieces. However, with enough ingenuity, a DIYer can make every piece of a gun from scratch. Constructing the barrel is an integral part of the gun making process.
Thick-walled iron pipe
Lathe
Metal-lathe attachments
Sand paper
Gun-dyeing kit
Cut a piece of thick-walled (.25 inches or more in diameter) iron pipe so that it's long enough to form the gun barrel. Make sure the diameter of the pipe fits your bullet snugly. Too tight, and the gun may explode in your hand. If the barrel is too loose, you'll lose accuracy.
Mount the pipe on the lathe and find a metal scribe with eight teeth. This will give you eight evenly spaced rifle grooves in the barrel. Set up your lathe to allow one dull twist of the scribe every 28 inches; most black powder weapons have this twist or slower. Run the scribe through the pipe two to three times in succession to make the grooves deep and smooth.
Use another scribe to machine threads into one end of the barrel so it will screw into its end plug. Run a deburring head through the barrel and threads. Use sand paper to smooth down the outside of the barrel and prepare it for dyeing.
"Blue" the barrel with a dye kit from any outdoor retailer. Let dry for at least 24 hours and move on to the next phase of gun construction.