Marc gave a good explanation.

Old skewl Mil dot has been surpassed. There's nothing wrong with it, as long as it has .5 Mil hashes in between the dots. Viper makes a good reticle, Bushnell Elite, SS, and both of Burris's Mil based reticle. The SCR Mil has more precise aim points, the Mil dot is faster to find where you want to be.

I've got the G-2 in the Bushnell Elite ERS 3.5-21 and really like it. It's based on .5 Mil increments vertically and horizontally. It's not too busy like the H-59 available in many manufacturers offerings. The fastest I've ran my range shooting with our holds is 200, 300, 400, 500, 600, 700, and 800 yards in 60 seconds. Much of that is the G2 reticle giving me definite holds for elevation and wind, as opposed to holding out in space for wind on a regular Mil based reticle.

The prs has a stage that's hold over only it's a bit challenging. It can be done without a tree but it's more difficult.

PRS Skills Stage 4 will consist of 1.5 moa targets at 200, 300, 400, and 500 yards. Start position 10 yards behind firing line, mag in, bolt back. Sound of the buzzer, the shooter moves to the firing line, assumes a prone position and engages all 4 targets with one round each from near to far without dialing any dope on their scopes. After the 500 yard target is successfully engaged the shooter must drop their magazine, move back to the start point to retrieve their second magazine and return to the firing line. Shooter will then reengage all targets from near to far shooting from the prone support side position. Round count unlimited 2 magazine minimum Par time 90 seconds 500 400 300 200 Firing Line 10 yds.

Yep but with running and then a switch to support side. Why a 400yd zero?

I dialed to 400, since it's kind of in the middle out to 700. It offers the easiest hold over at the 200 and 700 yard shots. I try to center up the holds. I don't remember my exact come ups, but this data is very close. I'm running my 168 A-max Bulk ammo, and in my rifle it's running about 2700 fps.

My come ups are (not exact, but VERY close), all in MILS:
200- .5
300- 1.3
400- 2.2
500- 3.3
600- 4.6
700- 6.0

Knowing my come-ups, I looked at them, and with a 400 yard zero, my hold overs/unders would be close to full mil and half mil holds, which look like this:

200- +1.7 Mil, (hold under) math- 2.2 minus .5= 1.7
300- +.9 mil (hold under) math - 2.2 minus 1.3= .9
400- Hold dead on (2.2 mils)
500- -1.1 mil (hold high) math- 3.3 minus 2.2= 1.1 mil
600- -2.4 mil (hold high) math- 4.6 minus 2.2= 4.6 mil
700- -3.8 mil (hold high) math- 6 minus 2.2= 3.8

If you look at the actual holds with a 400 yard zero, the holds are very close to half and full mil holds, which make it easier to hold. Also, the targets at 200, 300, and 400 yards are a 2/3 IPSC plate, which are a fairly large plate that cover at least .3 to .5 mil, so there is some wiggle room in there. If the target is .5 mil tall at 200 or 300 yards, then the hold over/under doesn't have to be "exact". The 4, 5, 6 and 700 yard plates are the more precise one's to hold for, plus adding in some extra wind as I was moving further out. I was holding left edge or just off left edge on the 600 and 700 yard plates, and using the reticle for the elevation. I like a simple mil reticle, and I think the Nightforce MLR is the best mil reticle ever made. I've never cared for the Horus style Christmas tree reticles, but they do work for many shooters.

Ok. All my rifles have a true zero of 100 yards.

I like the bushnell G-2. Fine enough not to obscure the target. A lot of mil dots and other ranging reticles are too thick. This is at mag magnification 3-12 elite tactical. The actual glass is a lot better than the pic. Its my iPhone looking through the scope.

Popped this axis doe right in the dome this January a few seconds after this pic.

I used to head shoot from time to time on meat hunts, but I quit the practice after seeing a doe running around with her bottom jaw 12" from the top. Looked like an alligator!
Someone missed her knoggin by a couple of inches and she suffered until I took her out with a shoulder shot. She was showing ribs and the wound was all black and infected, so she survived a while not being able to eat or drink.
I decided from that point that there were no more head shots for me, as the margin of error is too small and the suffering too great.