Strictly speaking, a carrier barrel is just as you stated, a 12 ga barrel bored out such that when tubes are installed, it weighs and balances equal to a regular 12 ga barrel, the primary purpose of which is to give you a matched set. A true carrier barrel will be pinned in the chamber to prevent a 12 ga shell from being loaded as they are generally too thin to use as a 12 ga shooter.
There are variations, I found that Kolar's carrier barrels (for their guns) were too light for tournament skeet (for me, at least), yet I still wanted matched weight barrels. So, they bored the tube barrel out to .750 or so (still thick enough to shoot a 12 ga shell, so not pinned, but reduced the tubed weight somewhat) and added weight attached between the barrels (mostly under the fore end) to the regular bore 12 ga barrel. Now they match in weight and balance, but are heavier than the "pure carrier barrel". Any reasonable combo of weight can be achieved this way.
Both Briley and Kolar will work this approach on any barrel you wish.
Chers
