Most of the information in this post was sourced from RCI's website, including their instruction manuals. Pictures and what I recall, are from my inspection of their product at the 2013 SHOT show. I cannot speak from experience in firing this product, but I did examine it at the show.Roth Concept Innovations's X-Rail is one of the newer solutions to the question of how to get more capacity in a tube-fed shotgun. Like some of the other extension devices, it relies upon a rotary magazine extension which allows shells to be stored in extra tube extensions instead of a straight line out the barrel. As of Apr 1 2014, it's available for:
Remington 870, 1187, 1100, and Versa Max
Benelli SBE, SBE2, M1, and M2
FNH SLP (requires additional replacement mag tube to work)
On most of these platforms, it allows the shotgun to hold 22 rounds with the full-size version, and 14 rounds with the compact version. One of their marketing points is that it reduces upward climb and perceived recoil due to the added weight on the weapon. The compact version weighs 2lbs empty, and the full version weighs 2lbs 5oz empty..
The X-Rail is notable for being auto-indexing - it uses an assembly of four extension tubes under spring tension, and as soon as one of its extension tubes is empty, the system will rotate to the next tube. The last tube has a longer spring which can push shells all the way to the chamber-side end of the magazine tube, but the auxilliary tubes have shorter springs which only push the shells to the end of the magazine tube. When an auxilliary tube is empty, the assembly rotates 90 degrees to place the next full tube in a position to feed the magazine tube. A valid 10 second description of the X-Rail is "a spring-loaded speed loader which automatically tops off your tube magazine from the front until it runs out of spare shells".
The system won't feed 3" shells through the extended tubes, but you can still use 3" shells if you aren't using the auto-indexing capabilities (hence the extra capacity only works when using 2.75" shells).
Per their instruction manuals, installation requires the removal of your original follower, spring, spring plug, and spring plug tabs. This may require modification of your original magazine tube via swaging or drilling out the tabs (indentations) which are intended to stop shells from trying to travel past the end of the magazine tube.
A side view of the full version X-Rail, installed on a Benelli shotgun. The metal housing attaches to the top of the barrel, and the magazine tube extensions run forward.
The other side of the full version. Note that the adapter, on this shotgun, occupies the space where the magazine tube would normally reside.
Underside of the full version. At the top of the image, one can see the feed ramp. One loads the X-Rail just like any ordinary tube fed shotgun.
This is the compact version, which is available in either a black tube like the one above or a clear version. This is a side view of the compact X-Rail. Due to the transparent plastic involved, it's possible to see the springs compressing as shells are fed into the X-Rail adapter.
The small piece I'm pulling here is the indexing pin, which locks the X-Rail to the primary tube. If you don't pull this pin, the X-Rail won't index, and it will behave like an ordinary tube-fed shotgun. When you intend to load shells into the X-Rail, you first completely load the main tube, then pull this pin so that the X-Rail can index, and then any additional shells being loaded will go into the three auxilliary tubes in sequence. Because the main tube's spring is much longer than the others (necessary so that it can push shells all the way to the end of the tube), rotating the barrel before loading the tube will result in the main spring being caught in the X-Rail assembly as it rotates, possibly damaging it in the process. There aren't any real engineering ways around this risk, so the indexing pin isn't a bad idea.
A view from the front end.
The X-Rail, with a few shells loaded.
The X-Rail, with a few more shells loaded. Note the "jaws" or "fingers" which attach to the top of the rail. They are tightened down via set screws, and then sealed with a thread fastener such as Lock-Tite. There are also set screws on the bottom, so that the X-Rail clamps on with tension from both above and below the barrel.