For factory barrels I mostly always used the original factory barrel boxes when I had them, or short shipping tubes bought cheaply for the pistol barrels, but sometimes free priority flat rate boxes as well; free postal Delta tubes (triangular - 2 lengths) or shipping tubes for carbine/rifle barrels; and the original boxes the custom barrels came in.
Even with my large assortment of TC's over the years the boxes didn't take up much room to store and save (no where near what the factory boxes for about 5 dozen rifles have anyway). Also kept most of my scope boxes as well for when they were resold, and the handgun boxes. Wish I had kept the hard-side rifle case boxes as well. I hate "making" boxes to fit, but will do it at any cost in time investment and materials when I have no other easy choice to make sure the item makes the trip safely. There's not much worse for a buyer than getting a new toy and receiving it damaged. My buyers soon learn I'll make the extra effort for them. What goes around sooner or later comes around.
FYI - Companies such as those that install custom window treatments (rods/blinds) or custom closet organizers are a great source for read made "gun friendly" shipping boxes that are usually free for the asking and often include packing.
Larry