Found out that the circa '94 Colt HBAR I own - but hated - has an upper that fits just fine on my newer lower. Both have .250" pins.
The old Colt Match Competion HBAR, complete with f#^&ing sear block, has a trigger that has a smooth "cinder block dragging on asphalt" feel. It had 3 pulls, one grind, one drag, and the final creepy one made it go bang.
Putting that 20" upper on the lower that has the
good trigger means I can get some more velocity, leave the scope mounted for varmint hunting and leave the Eotech on the M&P 16" upper.
Now for sighting in with Hornady Superformance! (that stuff in 53gr 223 is faster than 55gr 5.56)