Say the scope is a Vortex PST 6-24x50 settings would be 24-12-6


Here it is:

If a scope on the second focal plane has hash marks and they are correct at full power 24 (1 MOA) would they be accurate at half power 12 (1.5 MOA) and at the lowest power 6 (2 MOA)?



Thanks I'm just not sure if I'm thinking about correctly

Good question.  Tsg

If 24x = 1 MOA then
  12x = 2 MOA (half the magnification 2 time the spacing MOA)
    6x = 4 MOA (1/4 the magnification 4 times the spacing MOA)

You should check the calibration of the dial to the actual magnification level for the MOA you want.

The following video is for a mil dot scope, but the explanation is the same for MOA

Your thinking is correct qualitatively but not quantitatively - 1 MOA @ 24x is 2 MOA @12x and 4 MOA @ 6x.  There is also a "technical difficulty" as well from a "precision" point of view...  At max power, you just turn your zoom ring until it maxes out and you are on exactly 24x.  For power below that, you are guestimating the position of the zoom ring to get the magnification that you want.  If you had a calibrated target, you could take it to the range and dial until you get to precisely the point were the zoom and the MOA conversions match up and then place a witness mark on your zoom ring.  Then, in the field, using the witness marks should be pretty reproducible.

The best way to find out is to hit the range.

The best way to find out is to hit the range.

The easiest way to think about it is that every combination has to be equal.

For example if the hash marks on a MOA reticle subtend 1 MOA at 24X, then 24x1=24. If you are using 12x, then the hash marks will subtend 2MOA, because 12x2=24.  Similarly, if you are at 8x, then it's 3 MOA because 3x8=24, and at 6x it's 4 MOA as 4x6=24.

As a practical matter however all you really have to worry about is 1/2 the normal magnification for ranging and hold off purposes.  If you are shooting at long range, you'll be using the normal magnification (usually the maximum, but not always).  If you are shooting at a shorter range where you need more filed of view, then 1/2 the normal magnification should work fine, and you just have to remember that the hash marks are now subtending twice what they normally do - 2 MOA if they are normally 1 MOA hash marks and 4 MOA if they are normally 2 MOA hash marks.  You won't be giving up much to a FFP scope since at low magnifications under about half the maximum, a FFP reticle gets so small that it's hard to use the hash marks on the reticle for ranging and hold off purposes with any degree of accuracy.

The comment on using an index or witness mark is exactly right.   Nightforce includes an index mark on their SFP scopes for this purpose.  However, if your scope's zoom ring is not marked that way, you'll need to get some Redfield style sight in targets with 1" grid lines and then tape 2 or 3 of them together, aligning the grid marks top to bottom so you get a continuous tall target. Then carefully measure the distance and adjust the zoom scope so that the hash marks precisely subtend the correct distance.  Just remember that 1 MOA is 1.047" at 100 yards, so 20 MOA is 20.94", so if your target is set up at 100 yards, the correct distance subtended by 20 MOA on the reticle will be will be 21", not 20".   Similarly, 30 MOA will be 31.4", not 30".

If you can do so at your range you can also use a laser range finder to set the target up at 95.5 yards from the scope.  At that distance 1 MOA will equal 1".