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FNH SLP Mark 1 Tri-Rail Extension
 

While doing some research, prior to purchasing my FNH SLP Mark 1 shotgun, I ran across a forum thread regarding an optional picatinny rail.  This accessorty picatinny rail is a tri-rail setup and made by the folks at FNH.  It is designed to replace the factory sleeve that fits over the magazine tube.  The tri-rail extension tube is made for both the SLP and SLP Mark 1.  The part number for the SLP Mark 1 model is 3088929878.  The part number for the SLP model is 3088929876. 
 

A side by side pic of the tri-rail and factory sleeves.  Construction of the tri-rail seems to be quite sturdy.  The first thing I noticed was the weight.....it was more than I had expected.  It's not like it weighs a lot, but I noticed the difference as soon as I put the tri-rail on my Mark 1.  The fact that it mounts out past the end of the forearm does add to its affect on weight distribution.  A magnet doesn't stick to either the tube or the rails so most likely an aluminum alloy is being used.  The tube is machined (very nicely) from stock to provide for the three mounting rings that the rails are attached to.  The fit into the forearm is as good as it gets....nice and tight just like the factory sleeve.   There won't be any rattling from this accessory. 
 

A pic showing the end of the tri-rail extension tube.  The tube has MIL-STD 1913 spec rails so no problem hanging the wide variety of lasers, lights, etc. on it.  Each rail provides 5.5 inches of mounting space.  That is a pretty fair amount of room, in my opinion.  I can't imagine anyone filling up all three rails, with their "stuff", for a shotgun.
 

Swapping the factory sleeve out for the tri-rail sleeve is as easy as unloading the shotgun and unscrewing the magazine tube cap.  Slide the factory sleeve off and slip the tri-rail on its place.  Replace the cap and you are good to go.  It doesn't get much easier than that. 

When I ordered by tri-rail, I contacted FNH and they referred me to Bart's Sport World.  I called Bart's and they shipped it out right away.  I had found threads on the forums indicating the tri-rails were all out of stock.  Bart's  indicated a new order had just been delivered and so I caught it at the right time.  My shooting buddy bought one too and put it on his Winchester SX2....perfect fit.