Blueprinting

What blueprinting does:

Blueprinting aligns everything to the center line of the bore.  To get the most accuracy possible out of a rifle this is highly recommended.

Rifles from the factory are not perfect.  Every once and a while there will be a factory rifle that shoots very well but this is a rare case.  The reason for blueprinting is when rifles are produced the don't have perfectly mated surfaces such as the action face to the barrel shoulder.  This is also true for the bolt lugs and locking lugs in the action.  Normally on the lugs there will be more contact on one side than the other.

When you have uneven surfaces it causes the bullet to be canted slightly while its in the chamber.  So when the canted bullet is fired it is engraved by the rifling more on one side than the other.  By more we are talking about maybe 1/2 a thousandth of an inch but this can cause you to not be able to group within an inch at 100 yards.  Blueprinting basically straightens the bullet out so that it travels evenly down the barrel.

Conclusion

When rifles are manufactured, they don't have even surfaces which cause inaccuracy.  Blueprinting trues up the surfaces to make them center line of the bore.  The surfaces which are trued include; the bolt lugs, bolt face, bolt nose (remington 700), action face, internal threads of the action, and the lugs on the inside of the action.