The only true standard of a bullet's point-of-impact is the barrel. Laser boresighting is performed using an in-chamber, bullet-shaped laser pointing device, or a rigid laser device inserted into the barrel end. The laser points at the true point-of-impact using the barrel as a guide. Then, each aiming device is adjusted to the laser pointer at a given distance. 50-75 yards is typical. Much farther and you can't see the laser aim point. Much closer, and the aiming devices crisscross substantially at longer distances. The weapon laser, weapon light, iron sights, red-dot sight and scopes must all be adjusted to match the boresighting laser.
Keep in mind that bore-sighting gets you "on the paper", as in on the target. In order to completely accurize your rifle, you'll need to take it to the range, shoot it at the distance you desire, even with the ammo selection you prefer.
For $20 TacOpShop offers a boresighting service on any rifle we sell. Simply add the boresighting service to your shopping cart at checkout and your rifle will arrive bore-sighted.