I've seen the pics that everyone has, but what does an SPR actually consist of?  What are the defining characteristics?  I want to build one off of the RRA lower I've got.  It has a standard A2 stock, an Ergo tactical grip and that's it.  I plan on changin out the trigger to either an Accuracy Speaks or an RRA 2-stage.  I want to build the upper from the ground up as well.  I'm going to start with a stripped RRA A3 upper.  What do I need from there?

[url=http://www.ar15.com/forums/topic.html?b=2&f=134&t=141410]The SPR Story[/url]

Do a search using "SPR" in the AR forum and you'll see at least a dozen threads asking the same thing over the past 30 days.  The all lead to that thread for the answer.