Quote Originally Posted by KevDen2005 View Post
How come it is caliber-ed in .223 and not 5.56. I am guessing there is some historical lesson here that I am not aware of.
The early M16 (601 and 602) and SP1 rifles and magazines were all marked '.223'. The Air Force continued up until about 1968 marking theirs '.223', while the Army changed over to the NATO STANAG designation '5.56' sometime in 1967-68 with the adoption of the M16A1 (604). SP1s into the 1970's were still marked .223 - at least into the 40k# range, probably beyond.

Both the Colt civilian AR15 and Military M16 rifles shared the same components, including barrels. I've found no evidence that the chambers of the early SP1s differed from their military counterparts in that they were both designed specifically for the M193 5.56x45 cartridge.

For a time in 1963-64 the AR15 was the only production rifle chambered in the then new .223 / 5.56 available to the public. Remington didn't even introduce the "new" .223 to the public until 1964.

BUT, the .223 designation had been around as early as 1959 to differentiate the round from the .222, .222 special and .224 calibres then being experimented with by Armalite and Springfield. It wasn't standardized into what we know today as the M193 until about 1960-61 (?) by Eugene Stoner working with Winchester. About 14,500 prototype and trial rifles were manufactured and tested from 1959-1963 and all but a few hundred are marked '.223' and chambered for what would be adopted as the M193 5.56x45 NATO cartridge.