If you are talking about sighting in a new scope on your rifle, do not worry about a bore sighter. Spend the money on practice ammo!
Take ammo and rifle to the range and set up on something fairly solid - a chair, shooting sticks, etc at 25 yards. Get comfortable and fire one shot at the center of your target. Measure the elevation and windage between the hole and the center and adjust your scope 4 times the amount listed for your scope at 100 yards. (because you are 4 times closer)
Fire one more shot and repeat measurements and adjustment until you are within an inch of the center.
Now you can go back to 100 yards and you will be on the target paper but will need some fine tuning of your sight adjustment.
Have fun - practice lots!