The USMC M500A2 (Mossberg 500) is either being retro fitted or updated to the 500 Flex system. I'm a little unclear on that. In any case, there's a more recent designation, the MEK or military enhancement kit. It uses the Flex system for it's ability to be configured as desired with what looks like a 14" breacher barrel.
***EDIT***
I may be totally wrong about the 14" barrel after another look.