One more tip for boresighting by looking through the barrell: The farther away the object sighted on, the longer the sight radius and so the closer you will be to perfect at 100 yards. My all time best bore sight point was an insulator on a power line 3 miles out the window from my kitchen table. Nest the rifle in sandbags, center the insulator in bore and dial the scope to the insulator without moving the rifle.
Quick check back and forth from bore to crosshairs without touching the rifle and it is ready to go to the range. My second most accurate handy aiming point was the corner of a chimney 250 yards away. When I first started boresighting our normal was to boresight at 25 yards. 2500 produces far more accuracy.
As said, make sure the object being sighted on is CENTERED in the concentric rings of bore, muzzle, action, and it is pretty easy to be within 3 or 4 inches at 100, and often closer.