Quoted: Damn!! It's amazing how fast a "current" posting can disappear into oblivion! i think I found the answer to question #1 - small pivot pin vs large pin - but I'd still love some input on Question #2. Can anyone give me any help on this one? Thanks!!
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If question #2 is where you asked if this upper could be built into an SP1, the answer is; only if you can find an SP1 lower to mount it on.
Please understand, the Colt SP1 was a one of a kind type. The lower was totally flat sided, the rear ring is the early type, which you can't get on a factory lower any more, and Colt only produced the thing the way they did to dump some very early lower reciever forgings that couldn't be used to make M16s, and or M16A1s because of the flat right side.
I guess, if you're a master machinist, or are willing to pay one, you could theoretically convert a current lower to SP1 profile; but why the hell would you want to? It was a bastardized design in the first place. To my mind you'd be much better off getting one of the reduction sleeve type adapters from Bushmaster, or whoever, and mounting the thing on a current lower.
And oh, by the way, I've been an SP1 owner since 1975
The only reason I keep the one I have now, is that I class 3d the thing in 1979, and I'm counting on it to put my 9 year old through college in 10 years or so