Older AR15/new-upper compatibility?

[Walipala]

Hi folks. I hope someone out there knows about this stuff. I have an old Colt SP-1 (AR15). It's a 1970s era rifle. The original upper was replaced by a surplus M16-A1 upper at some point.

I have no idea how the original upper receiver fit or what the front pin/screw looked like, but with this setup it's got a screw with an eccentric (offset) shaft. This allows you to play the alignment a little. I suppose this was necessary due to the 'bastard' combination of civilian lower and military upper? This setup has worked okay---it just requires a little adjustment now an then.

Here is my real question: If I want to buy a new upper receiver-barrel setup for this rifle, what types are available? Would a Bushmaster or DPMS upper receiver fit?

I was thinking of getting a new-made A1-style receiver from one of the above mfrs and building a new upper. Being as this is a Colt, should I stick with Colt, assuming it's even available?

[VUPDblue]

The older SP1's were "large hole" receivers. Colt made receivers with a large front pivot pin hole. There are conversions available, and it sounds like you have one installed already, that allows a standard hole upper to be used. The "shim" is eccentric, like you describe. Now that you have that in place, you should be able to mate-up any upper you like. FWIW, the conversion pin you have is very old, and you can get a newer, better one. That's what I'd do anyway.

http://blackrifleworks.com/colt-items-c-63.html
The third item down is what you need.

[Walipala]

Thanks. That's interesting to know. And thanks for the link. You've actually answered a question I did not think to ask---is there an upper receiver that'll fit better than the one I've got now. I guess the answer would be "no", unless I could find an original early Colt smooth-sided upper like the one my rifle originally came with.