Howdy,

I'm a longtime visitor and brand new member.  Having got my first AR in July, I am already adicted and she has already "multiplied," just as I was warned she would! home
The lower is an Olympic from early 1994 (I checked their site).  

The upper is a pre-A1, ie. A1 sights, light barrel with bayo lug, no forward assist and no case deflector.  The forward take down hole is larger, requiring the use of a bushing to make things stay put.  is that what those "Colt conversion pin" things are for? maybe I need one.

The only markings I see are a "CM" just below the rear sight on the upper.  Is this a Colt "SP-1?"  Does that mean I would have a 5.56 chamber and 1 in 12 twist?

The smooth side upper, light barrel and A1 sights are what attracted me to this poor old girl, collecting dust at the gunshop.  I immediately imagined how sweet she would look after replacing the crappy immitation A2 handguards with a nice triangular A1 set (already on the way), an early-type sling and some good old TLC!  In other words, I plan to make myself a nice (very) early Vietnam-era rifle.

I checked all her guts yesterday and everything works under the hood.  I cleaned the bore and it appears bright and sharp.  I plan to do a complete take down and scrubbing with CLP today.

Any info and/or comments appreciated.

Thanks.

Your upper is indeed a Colt SP1 takeoff, and if you are going to use it on an Olympic lower, you will indeed need an adapter sleeve setup.  If it has the original barrel, there should be some markings on it, forward of the front sight; C MP and either nothing after, a C, a B, or "CHROME BORE", depending upon the age of the piece.

The offset mounting pins you often see mentioned are NOT what you need for this piece. They are for using an early LOWER with a small hole UPPER; the opposite situation to what you have.

Bushmaster, however, does make a kit that sleeves the large hole upper to match the small hole lower, and gives you a "semi" push pin type cross pin