The lugs are exactly the same width -- .354"

The 30-inch 12-gauge barrels weigh 3 pounds 2.1 ounces and the 29-inch 16-gauge barrels weigh 2 pounds 12.2 oz for a difference of 5.9 oz which accounts for all but 0.2 oz difference in the guns weights.

Getting this all going I tried swapping barrels and forearms. The 12-gauge barrels and forearm go on the RBL-16 receiver like they were made for it. The 16-gauge barrels almost go on the RBL-12 receiver, but I wasn't about to force anything.