Making a Nordic extension tube run flawlessly.
After much research here due to problems making my new Nordic +3 extension tube run in a 930 Tactical, I am posting how I achieved success.
Problem #1: Stock follower and spring get caught on the end of the stock tube at the joint between the two tubes. Ryan at Nordic recommended a lead in chamfer on the factory tube. Used a rubber abrasive polishing wheel and polished a smooth chamfer into the end of the tube bringing it almost to a point in thickness which prevents the spring or follower from hanging up. Ryan and others who posted here commented on the value of a Nordic low drag follower which I added. The slight modification to the factory tube combined with the low drag follower eliminated this issue 100%.
Problem #2: Too much spring pressure with the recommended spring lengths causes failure to feed out of the tube and double feeds. The Nordic spring has a significantly higher rate compared to the stock Mossberg. As I continued to cut down the spring an inch at a time it progressively got better until I could get 6 out of 7 to feed 90% of the time with a 25" overall spring length. Number 7 was a 50/50 on feeding. Started analyzing the action of the shell stop mechanism and the marks that it left on the brass at the base of the rounds and decided to disassemble and improve this part. I found that this mechanism is the most critical part of making this gun feed reliably. Removal requires disassembly of the barrel, bolt and slide, but is not really that difficult. There is a clip hidden in a slot inside the receiver that retains the pin holding the shell stop mechanism in place. When you have it out, place the flat end that contacts the shell base on a piece of sandpaper on a flat surface and recut it until it is truly flat. From the factory it is not perfectly flat and has some sharp edges that grab the shell base with too much force to allow the shell stop to slide sideways as the bolt is opened. I finished with a polish with the rubber abrasive wheel to the flat face and lightly touched all of the edges up. Thoroughly cleaned the mechanism and the pocket that it rides in. Also noted that on the top edge of the elevator where the tail of the shell stop mechanism makes contact had a wear mark. The elevator is a steel stamping and the edge is not really that smooth. Took a fine file and flattened and polished the top edge of the elevator where the shell stop tail needs to glide under it. Reassembled with no further changes to the spring length and I have 100% reliable feeding and no doubles, and this is when working the action by hand. There is only so much spring pressure available in the shell stop mechanism and you have to remove any resistance to it's proper functioning. I wish that I had started with this step because I could have probably gotten 100% feeding with a couple more inches of spring. The 25" spring does not have enough pressure to blast out a single shell in the mag tube when unloading by raising the elevator and depressing the bolt release so I have to cycle the last round out with the action. Second and subsequent rounds always unload without a hitch so I could have used just a bit more spring pressure for popping out the single last round.
Lots of guys on here said that some problems experienced when hand cycling disappear when shooting, but I believed that I should make it work properly on the bench to achieve flawless cycling when shooting. Many thanks to Ryan at Nordic for spending time on the phone troubleshooting and making recommendations.