Thanks, Dan. I knew about the different diameter takedowns, but had forgotten that they were drilled on different centers. Now that you say it, I recall cam-looking pins that were marketed to allow mating uppers and lowers with the pin issue.SP1 BCs were from M16 production, but their bottoms were machined back so they could not trip auto sears. SP1 uppers are somewhat different from M16 uppers. Not only do the front pivot holes have different diameters, but they have different center points too. Also, SP1 uppers are not machined out on the bottom for auto sear clearance.