Someone else can probably confirm if your NID has short chambers, but I'd be surprised if it did. In any case, you can buy 2.5" 20ga ammo from a number of places online--RST among them. You'll get opinions on both sides of the fence on whether or not it SHOULD be done, but it absolutely CAN be done.
I've had Mike orlen, in Amherst Mass do this for me on several guns with very good results, I think it costs $45 per barrel.
Many people shoot 2 3/4" lower-pressure reloads in short chambers since the pressure increase is not "off the chart", relying on the already-low pressure ammo to keep them below the pressure the barrel was designed to handle. I'd be interested to hear what more knowlegable people than I have to say about doing this in a 20 vs a 12...is there a difference?
