The other day I was cleaning my 930, had all the pieces scattered about my living room floor and accidentally stepped on my factory magazine spring, bent it and ruined it. So before I started ordering things and getting into trial and error, I decided to take the Nordic Components spring and magazine follower out of my 870 and see if they worked in the 930. They feed perfectly together and I still have a 7+1 capacity. I loaded it with 7 a handful of times and cycled all of the shells out, mostly testing to see if the spring was long enough to eject the last round and it was. I started loading one at a time into the tube and cycling it with the same results. The spring was cut to 25" originally and it has fed 100% in my 870 with a custom 7 round magazine so I see no reason why it wouldn't work in the 930. I've kept my 870's magazine loaded with 7 for probably four months now and it's compressed to 24" but still feeds perfectly in both guns. I've read about people with springs cut as long as 35" (my factory spring is 30") in their 930s and the way I look at it, the NC is much stronger than the factory spring so I don't need as much of it. If I've made some kind of gross error here and the shotgun fails miserably as some point in the near future, I'll update everyone on it but like I said, I've had success with an almost identical magazine with this spring and follower combination so I don't think that's going to happen.
A 930 SPX has 18 3/4" of total magazine space. A typical, unfired 2 3/4" shell is about 2 1/4" so that's 15 3/4" and that leaves 3" for the spring to compress. The Nordic Components follower is longer than most others but is still only 1 3/16" long.