Thanks for responding. Gotta admit. That’s a heckova shot for a guy with no practice and a walnut-sized tumor in his noggin. The Tower would be approximately where a 24th-25th street might be if one existed.
He had had plenty of practice in prior years and in the Marines. He mostly used a Remington 700 he already had. After the first ~20 minutes of his spree, return fire - mostly from civilians - kept him so suppressed that he could no longer fire accurately and had a massively reduced rate of fire. All of his killing from the tower was done in that first 20 minutes.
Also, I forgot that he actually made a longer non-lethal shot. He wounded Billy Snowden in the shoulder while he took cover in a barber shop doorway, some 500 yards out.