New production 930JM shotguns use our MXT extension tubes. Capacity can be increased by using either an MXT Coupler to add an additional extension tube or by installing a longer MXT extension tube.
However, older 930JM shotguns use our older EXT extension tubes, which have different thread specs for the tube cap and will not accept an MXT Coupler in the same way as an MXT tube.
There are a few options for adding capacity on these shotguns.
1. Buy an MXT tube of the same length as the existing extension (either +4 or +5) and a new MXT tube end cap along with a new spring. This will convert your EXT to the current MXT extension.
2. Replace the existing factory EXT extension tube completely with a new MXT extension tube of the desired total capacity. This will also require a new cap and spring.
3. Purchase an MXT coupler and new extension tube. Remove the EXT extension tube from the EXT nut, and thread the MXT extension into the EXT nut. Add the MXT coupler to the end of the MXT tube, then thread the original EXT extension to the end of the MXT coupler and add the original endcap to the EXT tube. This is a more affordable option, however the MXT extension will require a different endcap if you choose to remove the EXT extension, and would significantly decrease your magazine capacity if you used only the MXT extension.