I had the problem of shells hanging up in the magazine tube of my 930 SPX. I was able to fix the problem following some of the suggestions in this thread. I just wanted to put what worked for me out here.
The reason for the hang ups has already been mentioned earlier in the thread. The factory Choate extension doesn't fit snugly in the magazine tube. Either the follower or a shell base gets stuck in the transition between the extension and the tube. Here is what I did to fix that:
Start by changing the follower. The factory follower has sharp edges, which makes it more likely to get stuck. I went for the GG&G Mossberg 930 Follower. There has been suggestions of using a Wolff spring (part #17905) in the forum. I tried that too. I ended up trimming about 1.5 inches from the said Wolff spring to make it fit. In my case, it did not help with the hang up problem. I went back to the factory spring.
Next, I sanded down the teeth on the Choate extension with 320 grit sand paper. Put the sand paper on a smooth surface. Press the Choate extension on the sandpaper firmly, and pull in one smooth motion. Lift, go back, and repeat. Rotate the extension slightly (to prevent uneven sanding) before each pull. It took me maybe 10-15 minutes to sand down the teeth. After this step, the Choate extension could screw in half a turn more into the magazine tube. I could still fit all 7 shells in the tube.
Last, I put a 30 degree chamfer inside the magazine tube end. If you haven't done machining on a firearm before, it is probably best to have a professional gunsmith do this. The chamfering bit I used is a Yonico 13905q (30 Degree with 1/4" Shank). You can find it from various vendors online. Stick some tissue inside the tube to prevent metal scrap from getting in. Place the magazine tube on a block so that it is level with the chamfering bit. Generously lubricate the end of the tube and the chamfering bit. Seat the chamfering bit in the tube. Hint: If the bit has seated properly in the tube, it doesn't wobble while turning. If it keeps wobbling, adjust the angle so it seats properly. Otherwise you'll have uneven chamfer. The magazine tube has thin walls, and I realized very quickly that the bit could chew it up. So I had the bit rotate in the wrong direction (i.e. rubbing against the tube instead of cutting into it) at moderate speed.
After all this, I tried loading & unloading the firearm many times, and put around 100 rounds at the range. No more hang ups in the magazine tube. Hope this helps people with a similar problem.