Well bore sighting is a no-go on the M1A, so I would start at about 25yds. But remember, at that yardage the adjustments are going to have move 4x what they are marked. I.E. if your impacts are 1" high, instead of dialing 1 minute of angle down, you will have to dial 4 to get it in the bull. After you get it close at 25, hand a fresh target at 100(this will avoid confusion). Now shoot again. The rifle will probably be shooting high, dial down according to the elevation dial, and left or right according to the windage dial. Once in the bull, fire a 3-5 round group to confirm zero. If it is there, loosen the tree set screws on each dial(windage and elevation) and lift off. Set each one back on @ "zero" and tighten the set screws. I also mark the closest line(revolution counter) on the base of the turrent(non-turning part), with red sharpie, so as not to get lost on the erector.