I have installed a Nordic +1 extension on my 590. The one thing that I did notice is that with a standard style follower [the metal factory "cup" style, and clones thereof] the spring could bind up, stopping rounds from feeding. [Exactly the same issue occurred on a Benelli Nova that I had]. It seems that there must be either a rough spot or a small gap when the extension tube is screwed onto the magazine tube, and the compressed spring catches in there. I swapped out the follower and installed a plastic one with a tail on it, such that the tail acts as a spring guide when the spring compresses. Since then I have had no problems with the mag spring binding. NB As I am in the UK my 590 barrel actually has a 4" extension welded on to comply with UK law, so the +1 extension is still shorter than the barrel by an inch or so.