I have noticed a lot confusion online about the Colt AR15 as it became the SP-2, or the AR15A2. People assume the “A2” means that the uppers should have come originally from colt with the familiar M16A2 upper everyone knows. This is a understandable assumption but it’s wrong. Colt always used up old parts from military contracts before switching the civilian rifles over to the newest military type parts.
The first SP2( AR15A2) was still very much like the SP1. It’s actual internal Colt model number is the R6400.
Really to put it simply, it is the old SP1 with an A2 , government profile barrel and delta ring. It has a 1/7 twist with the new FSB and front post, A2 hand guards, buttstock and pistol grip. The upper and lower receivers are the same as the Sp-1 except for the roll mark.
As Colt ran out of the slick side uppers , they used up their remaining M16A1 uppers.
The next version of the SP-2 rifle has is exactly the same except it has a M16A1 upper receiver with forward assist. Colt model number R6401. Barrel is A2 milspec barrel and furniture etc.
The last version before the switch over to the true A2 upper receiver is the R6500.
The R6500 (SP2) uses the Canadian C7 type upper. This is an A1 upper but with a case deflector added and using the newer button forward assist. These uppers are seen on some LE and military carbines bought by the US Gov for SOF troops as well.
After these were running out or about to run out,the R6600 and R6550 models hit the market. Those guns uses the A2 upper with heavy barrel and government profile A2 barrels respectively. The lowers were still slick side SP1 type lowers for a while before transitioning into the now universal A2 profile lower.
Above is the typical rollmark of an SP2. All rifles used the double screw head front pivot pin that is larger than the milspec current front pivot pins used by Colt. The older large hole pins was a change forced on them by the government before they were allowed to sell to the civilian market. Adapter pins are available that lets you put large hole uppers on milspec lowers and vice versa if you get one of these old uppers and want to put it on your new lower or old lower on new upper. You can also buy pins to replace the screw head pins on the upper to make it easier to take apart or just to look better.