So,took the plunge and picked up a Wilson combat One Shot Tactical Magazine Extension with Sling Mount, for the M870/1100/1187 to try it on the mossberg 930.
It fits,kind of,I can't get it tight enough ,so I add a Sling Mount Plate, Vertical, for ,again the 870, this works very well because the detent on the sling plate should help keep the extension in placeand the "U" cut out at the top to keep it from turning.there's still about a 1mm or 2 of play in the forearm.
So what to do about that,A washer or maybe another sling plate,if I go with the sling plates the slots for the sling will need to be cut off.
Just need to find somebody to make a single part/adapter for this.
So,I tried this with snap caps to see how it would work untill I can get out to a range,it seemed to work O.K. just cycled it by hand ,no problems.using the mag spring and follower that came with the gun.the HiVis one that came with the mag ext was binding on the last rd for me.