No offence but this belongs on the forum and not PM...
You asked about the specs of the AR-15 SP-1 carrier and if it were possible to convert a non-SP1 carrier to SP1 specs... I used to have the exact specs for the Sp-1 carrier on my website (
www.Quarterbore.com but it seems I can't find it now. I can measure one of mine when I get a chance...
For what it is worth, I understand that current DPMS carries may well be SP-1 spec and they may work fine with a lightning link. I have not ordered one or used one myself but I have seen were people have said that they work...
As for the Bushmaster or just about any other AR-15 carrier, yes, you can modify it to sp1 spec... Here is a pic that shows a Bushmaster AR-15 Carrier (top), Colt SP-1 Carrier (middle) and Colt M-16 carrier (bottom).
It's just a matter of getting the trip area milled square and to the right size. You could modify an AR-15 or M-16 carrier but obviously it would be easier to do an AR-15.
Here is a guy that can do one for you assuming you have a legal LL...
http://www.m60joe.com/