FACTORY CAPACITY: The Mossberg 930/940 JM Pro Series shotguns come factory equipped with Nordic Components +4 (22″ barrel) or +5 (24″ barrel) magazine extensions. The capacity of the factory magazine without the extension is 4.
To expand the capacity of the JM Pro, the extension tube itself may be replaced with a different length, or an additional extension tube may be added with an MXT Coupler. The same extension nut and end cap may be reused, but a new spring will be required.
NOTE: Prior to 2016, Mossberg used our older EXT style extension tubes. These can be identified by the lack of a barrel clamp groove on the exterior of the extension tube. If your JM Pro has the older EXT tube, the existing extension nut can be reused, but a new end cap will be required if replacing the extension tube with a new MXT model. If using the MXT Coupler, the new MXT tube will have to connect to the extension nut; the existing EXT tube can be used attached to the coupler with the existing end cap.
The 930/940 shell stop can be very sensitive to magazine spring tension. We do not recommend using an HD spring in the 930.