Let's just deal with elevation adjustment to make this discussion easier.
There are basically two ways to adjust for elevation in the long range rifle game:
1. use the elevation knob to "dial" elevation
2. use marks in the reticle as hold-over points
You can also use a combination of the two methods, but let's just deal with one or the other for now.
When the elevation knob is used to dial elevation (method #1), each "click" of the turret corresponds to one unit of angular measurement. Typical values as 0.25 MOA, 0.5 MOA, 1 MOA, or 0.1 MIL. The Leupold M1 series is 0.25 MOA; the M3 is 1 MOA; the new M2 (on the 1.5-5x) is 0.5 MOA. Nightforce scopes are generally 0.25 MOA. Schmit & Bender offer 0.25 MOA and 0.1 MIL. US Optics offer 0.25, 0.5, and 0.1 MIL as far as I know.
To "dial" elevation, you first range the target using whatever method you want. Having your spotter shoot a Laser Range Finder (LRF, such as the Leica LRF1200) at the target is a great way to get this number. Then you consult the trajectory table for your load & rifle. This table is indexed by range and tells you how much elevation is required for each distance. You dial this elevation amount on the elevation knob, and then use the primary reticle aiming point (ie, the crosshairs).
Here's the data-card I use with my AR10 for our
Sporting Rifle Matches in Raton:
Code:
* 168gr FGMM 2575fps
sight 5000' shoot 6800'
ELEV 10 WIND WIND"
z109zz0.00zzz0.50zzz0.6z
*200**1.50***1.25***2.6* WIND
220 2.00 1.25 3.1 from
240 2.50 1.50 3.7 LEFT
260 3.00 1.50 4.4 dial
280 3.50 1.75 5.2 LEFT
*300**4.25***2.00***6.0*
320 4.75 2.00 6.8
340 5.50 2.25 7.7
360 6.00 2.25 8.7
380 6.75 2.50 9.8
*400**7.25***2.50**10.9*
420 8.00 2.75 12.2
440 8.75 3.00 13.4
460 9.50 3.00 14.8
480 10.25 3.25 16.2
*500*10.75***3.50**17.8*
520 11.75 3.50 19.4 WIND
540 12.50 3.75 21.0 from
560 13.25 4.00 22.8 RIGHT
580 14.00 4.00 24.7 dial
*600*14.75***4.25**26.6* RIGHT
620 15.75 4.50 28.6
640 16.50 4.50 30.8
660 17.50 4.75 33.0
680 18.25 5.00 35.4
*700*19.25***5.25**37.7*
720 20.25 5.25 40.2
740 21.00 5.50 42.8
760 22.00 5.75 45.5
780 23.00 6.00 48.4
*800*24.25***6.00**51.3*
A variant of the "dial" method is when instead of consulting your ballistic card, you have marks on your elevation knob which indicate which position corresponds to which range for the load you're shooting. So the number of clicks required for 300 yards might have a "3" or "300" marked on the knob at that position. These are called "BDC" or "BDC cams" and are specific to a certain load out of a specific caliber and barrel length.
The second method is to use marks in the reticle for hold-over. This method is identical to the "dial" method, except that once you consult your ballistic table, you are given a position in the reticle "below" the primary aiming point which demarks the amount of angular hold-over required for that range. Similar to the BDC cam, the reticle can have identifying marks in the reticle to tell you which hash-mark corresponds to "300 yards" and so forth.
A mil-dot reticle can be used with the second method ("reticle BDC") because the mil-dots below the primary cross-hair demark one mil per each, but it's kind of a coarse measurement.
The Horus reticles have a matrix of mil measurements below the primary aiming point, which makes them particularly well-suited to the second method ("reticle BDC"), except that you have to know how your load shoots in tenth of a mil measurements. On most of the Horus-equipped scopes, you can also dial elevation, or use a combination. The Horus reticle will really come into its element when engaging multiple targets at multiple distances "on the clock", or when shooting distant movers.
Hope this helps
Zak
ETA:
I realized I missed a couple questions. The Horus system is NOT tied to one load. Even Leupold M3 (1MOA) click knobs with the BDC on them are not tied to one load; you'd just ignore the yardage/meter markings if using a different load.
Most of the scopes you can get the Horus for will be as good or better than the Leupold. This is true for the US Optics and SB; I don't know about the others. But bring lots of $$$.
