I presume that you are talking about the "Companion" tubes that Briley makes to convert your double to a smaller gauge?
I have a set of 28 ga tubes for my remington O/U. They do make the gun balance differently as they add weight to the barrels. In some guns the tubes tend to rattle, particularly if you have open chokes. To solve this in my O/U I have to install full choke tubes in the 12 ga barrels to hold the tubes securely.
If you have several guns that have the same length barrels the Companion tubes are a good idea. If you are only interested in converting one gun consider having Briley make a set of their titanium tubes expressly for your gun. these tubes are much lighter and the choke tubes are easier to change while the tubes are in the gun.
There are companies who will sleeve the bores on older guns. This is most often done with older Damascus barrels that are not considered safe to shoot with modern ammunition. The sleeve is permanent and will make your 12 ga into a 16 or 20 ga gun. the advantage here is in the weight savings. Sleeving is more expensive than knock out tubes like the companions.
In short, the Briley tubes add weight to the barrels and affect the handling of the gun.
Geoff Ross
