What do they stand for and the differences in them?  I know RIS  = Rail integrated System?
RAS?  SPR? I read it DOES NOT mean Special Purpose Rifle.  So what is it?

Rail Interface System, Rail Adapter System, Free Floating, thought SPR meant just that..now I dunno.

RAS = Rail Adapter System
FF RAS = Free Floating Rail Adapter System
RAS II = FF RAS that attaches to front part of flattop rail instead of barrel nut.

SPR does stand for Special Purpose Rifle.  Based on the M16.  There are two versions of the SPR.  Both have FF handguards.  Mk12 Mod 0 has a carbon fiber handguard and the Mk12 Mod1 has a Knight's aluminum handguard with integral rails.  (According to Jane's Military Small Arms and American Rifleman)

Go here for more info on the KAC equipment:

[url]http://www.quarterbore.com/kac/index.html[/url]

SPR ( does mean spec. purpose rifle ) and have been type classified by the Navy Dept. at Crane as the MK-O and Mk-1. RIS-rail interface system, RAS-rail adaptor system, FF RAS- free float RAS, RAS-2- free float and grabs one nothch on receiver, SIR- selective intergrated rail.
Jack

There are still some who insist that "SPR= Special Purpose Receiver", which it was originally. "Receiver" changed to "Rifle" when the "Special Purpose" application spread to other types of rifles, such as the Rem 700, ect.