Saw that thread in the General Section where a guy put together a AR15  for $391 with a surplus M16 upper on a new lower. I want to do the same thing as he did, i just dont know all the details behind the different size pivot hole. I will be buying a new lower, not sure what brand. What size  hole do new lowers have? And what upper will fit it? Will surplus M16A1 uppers fit new lowers? And then the SP1 is the one that wont fit new lowers? I dont want to be stuck with a upper and a lower that wont go together. Thanks

Some older Colts COMMERCIAL models(i.e.SP1) use a large diameter pivot pin as opposed to the standard milspec (small) diameter pins that everyone else uses.

If it's a surplus M16 upper then it will have the standard milspec size pivot hole.

Colt is the only company that used the large diameter pivot pin on their rifles, but they stopped doing that a long time ago, so all newly made lowers including Colts will have the standard pivot pin hole.  

This should only be an issue if you are buying a older Colt commercial lower or upper.  



 

OK, thanks