This and alot of other Mossberg facts can be found in our FAQ Forum.1. Many people think the Mossberg rib is loose. The Mossberg rib is attached to the barrel at the rear with a cross pin. Along the length of the rib it is attached to each post by fitting into grooves cut into the tops of the posts. The rib has a channel underneath, and rides on top of the posts, secured by its two flanges riding in the grooves of the posts.
When a Mossberg barrel expands or contracts from the heat or cold, the rib is free to slide along the posts. Rather than being slightly bent from the rib expanding or contracting at different rate, the Mossberg barrel is free. It's a neat idea.