I've noted that the #38S-EX is 14" long and would thus extend out to create a sizeable rail; would it be stable enough to at least make a top rail comparable to that on the S.I.R. civilian version?

I am unsure about an M4 or AR15 but I did it on my ZM LR-300.  I had to fabricate a brace for the front to prevent it from flexing though.  Brace was a modified NC STAR soft aluminum picatinny short rail that broke on me.  Modified it using a dremel tool.

 I hear rumors that the #38 S-EX won't fit over a set of M4 handguards (double heat shield) but will on standard carbine HGs.

[img]http://www.imagestation.com/picture/sraid67/p3e871fb9982e11cc9705c3a2bfcdf5a7/fbc92c31.jpg[/img]

[img]http://www.imagestation.com/picture/sraid67/p70cd64728bb8fd7325f4122e3a8914c4/fbc92c3c.jpg[/img]

[img]http://www.imagestation.com/picture/sraid67/p7a9260020df2c5f9dd956e3472bc8e2d/fbc92c43.jpg[/img]

Love that stock!!!!
 

I can't see the brace you talked about.  Is it on the end of the receiver or is it just not in place for the picture?

Thanks about the stock.  I got the idea and help from THEO, he modified an ACE stock for his LR-300 and then told everybody else how to do it!  Great guy!  hat is what I love about the LR-300, NO BUTTSTOCK SPRING & nice fold away buttstock.

 The brace on the front of the rail:
[img]http://www.imagestation.com/picture/sraid67/pd52d828bbb45cf9df173341329914bb1/fbd31dbc.jpg[/img]
[img]http://www.imagestation.com/picture/sraid67/p5af895cbd0bae1990257448a639ff18e/fbd31dbe.jpg[/img]

 Edited to add:  There is a small screw hole on the front of the rail to hold a "bush breaker" to keep objects from getting under the rail & handguard.  You should be able to modify something to give the rail rigidity.  I had to use aluminum, as the brace rides atop a sheetmetal covering for the op-rod spring (in lieu of buttstock spring) and get very hot, so plastic would not be feasible.

 Maybe on an AR-15, one could use some rigid plastic similar to handguard material to make a brace with...?

The 38 S-EX comes with a metal brace that fits into the hole on the standard AR handguard.
At least mine did.

It's actually a brush guard...  the little metal thingy that extends down off the front of the #38 super extended.