So you want to create a insert barrel but with 14" inserts and in a twist lock?
6"-8" is the acceleration zone, and while i do prefer 1 piece barrels, the logistics of having inserts like that is going into the ludicrous state. It would be actually less costly to have 1 piece barrels and use a twistlock adapter(but have it so you can remove it like a TW barrel).
Twist locks when new some 15 years ago always had that +$10 to the price because of the extra material or the non-standard machining. They are also heavier(some people do try to cut weight on their guns). Though the latter plastic detent was incredible over the piano wire ones.
Take a look at a Vector barrel. It is a twistlock of sorts. That is more to what you want to do. Now getting that onto a modern gun, that is another story.