Tim, the only way to know barrel wall thickness for certain is by directly measuring all areas of the barrels. With SxS double barrel tubes, this means that no shotgun barrel wall thickness tool can measure the areas between the ribs. On the other hand, the area between the ribs is more likely, but not guaranteed to be thicker than an area at the same point from breech or muzzles that is situated on the outer left or right sides. The external areas away from the ribs are finish struck and polished at the factory, and they may have additional striking and polishing done afterward if the barrels are repaired or refinished.
If you look at photos of SxS shotgun barrels that have suffered a burst other than a chamber burst from an overload, etc. the damage is typically a split or bulge accompanied by splitting, and it virtually never is located right along the ribs, because wall thickness is generally a bit thicker there. Even when the cause is an obstruction like mud or snow, the failure is still most likely to initiate at the weakest point radially.
The reason for this is because once the tubes are joined, no further external striking of the tubes can be done between the ribs. Also, the area between the ribs is not subject to denting, so there is no chance that repairs that involved striking would create thin spots. Internal polishing or honing is likely to be fairly centered in the bores, but there is also no guarantee that the operator of the hone didn't exert force that was uncentered. So there are just too many variables to simply subtract I.D. from O.D. and think you have an accurate wall thickness. Even people who buy barrel wall thickness measuring tools seem to have a lot of problems getting consistent and accurate readings. I'd guess the vast majority of shotgun owners have no idea what the barrel wall thickness of their guns actually is. For the most part, ignorance is bliss. But if you have doubts, it would be prudent to have them checked by a competent gunsmith.
