I picked up a Colt SP-1 rifle last summer, a 1979-era rifle. I LOVE this rifle!!! It's been shot some, but just enough to break it in. It has caused me to re-think much of what I previously thought of AR's, with regards to reliability, or lack thereof.
My first AR was a post-'94 Colt Lightweight Sporter. It was a total POS!! It wouldn't reliably feed worth a damn, and I traded it off in disgust. I got a Colt blue label Govt. Carbine to replace it, and kick myself daily for ever letting that one go.
This SP-1, however, will feed anything I give it, from S. African, Guatemalan, to(gasp!!) Wolf!! My oldest daughter and I are on our second case of Wolf 55gr FMJ. We had to replace the hammer soon after getting the rifle, and the extractor spring was a bit soft, as it was probably the original. We have had NO jams using Wolf, nor have we had any lacquer-related problems.
My daughter has laid claim to the rifle, and has done a few minor alterations to it. It wears A2 front HG's, an A2 pistol grip, and an A2 buttplate, although she likes the length of the A1 stock better, so she has kept it. She can kill those offending bowling pins @ 100 meters with it regularly enough to get bored with it.
If the SP-1 you are looking to buy is NOT NIB ,then shoot the hell out of it!! They are an outstanding rifle, though I wish it had the 'hump' brass deflector. I have to use the little plastic clip-on deflector when I shoot it, but wish I didn't have to.