My first AR15 was what everybody now calls an SP1. (At the time, the choice was standard or collapsible stock- that's it. I don't remember anyone calling it the SP1 until later.)
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It's still my favorite AR configuration for just general shooting. It's lighter, has simpler sights, I like the feel of the handguards, and it can do most of what I need one to do.
I think it's best described as the Vietnam-era M16 style. The obvious features are the triangle handguards and windage-only adjustable rear sight, but they also have:
-Lighter weight, 1-12 twist barrel. As said already, not very useful for the heavier bullets. I have shot a lot of 60 grain Hornady bullets, but have not tried anything heavier/longer. since most available ammo is still M193/55 grain, it's not a bad handicap for general shooting.
-Pistol grip wihout the finger groove. I like it better that way myself.
-Stock is shorter, and has a buttplate with more rounded edges. Not a big deal.
-The material the stock and handguards were made from was different. It's a little more "brittle" feeling, but I wouldn't call it easily broken. Although there are plenty of Vietnam War pictures of taped-up handguards, that's a different enviroment than what mine has been through.
They changed over to a sort-of M16A2 configuration starting about mid 1984. Some of the first "AR15A2s" had a mix of SP1/A1 features and A2 features like heavy A2 barrel and A1 sights. I imagine it was a case of using up the parts on hand.
All "SP1" AR15's should have the full bolt carrier, small diameter trigger and disconnector pins, and large diameter foward takedown pin. All I have seen have a serial number starting with "SP", like "SP100000". But I don't know if that's true with all.
You might want to watch for these things, because there have been a lot of "SP1" uppers for sale over the years. Anyone can take any AR lower and mount an SP1 upper on it. It's just as useful (or not, depending on your view), but just won't be "genuine"- if that matters.
From what I've seen, SP1s have been the most "reasonably" priced of the AR bunch, so there should be little reason to "fake" one to get more money. But if you want a "genuine" one, those some the things to look for.