Re: spx rail prevent sighting w/ 28" vr fiber optic barrel?
I dont own a 930, but I'll give you my experience with rails and hopefully someone else in the "know" will bail me out if I open mouth, insert foot.
Rails, even when grooved usually arent deep enough. Typically if you try to sight through a receiver mounted rail, the plain isnt long enough to be accurate. I dont like swapping between the 2 either, but the rail removes fairly easily. Just keep track of the little screws.
On the extensions, you'll have to read through the Nordic Comp posts. Dan has detailed just what does what. There are also larger, (or longer) caps that slighly extend the extension which may get you that extra shell and make the extended tube even closer to the 18.5.
The thread on the nordic Comp parts is in this section somewhere...