[edit, title updated per suggestion. Some updates and additional calculations to be performed soon. Will produce a similar graphic for IMI 62 gr M885 ammunition, and some tables of numerical values as requested].
It's a common question, and one I'm revisiting. After two hours of analysis with a ballistics calculator, I decided to share my own personal assessment. Your goals and applications may be different - there is no One Correct Answer.
Scenario: RRA 1/9 16" Middy with an EOTech XPS2-0 mounted on a LT-110, for a sight hight of ~ 2.875"
Ammunition: Handholds to simulate M193: 55gr Hornady FMJBT, 28.0gr BLC2, 2950 fps muzzle velocity* (
*Technically M193 should have 3132 FPS out of a 16" barrel, though I've never seen this.)
Conditions: 1150' elevation, 32 °F local temperature.
A spectrum of zero distances were considered:
25, 50, 75, 90, 100, & 200 yards
My goal was to have a relatively flat profile. I also wanted my zero to be within about an inch at 100 yards. Trajectory was only considered out to 200 yards. I know many prefer to assess a 300 yard trajectory, but for a light carbine with a holosight, I do not. Each of the zero distances were entered into the Android ballistics calculator "Shooter", which was used to model trajectory. Results are shown in the below graphic.
Discussion:
The 25 yard zero and the 300 yard zero's both were rejected. >4" high at 100 yards was not acceptable to me. The 25 yard zero was particularly interesting. My thought was it would be a good close quarters/indoors zero (i.e. <15 yards). It didn't really shine. For indoor work (<10 yards), it's still about as off as the others, at 2" or so. At 100 yards it's ridiculously off. I finally concluded that trying to get a good zero that also worked indoors really wasn't practical.
The 50 yard zero was also rejected. Too high over the range spectrum.
The 200 yard zero was getting closer, but was rejected because it was greater than 1" high at 100 yards. Read below, because it ends up really close to what I selected.
I like the 75 yard zero. It's about 1.5" low at 25 yards (no worse than most), 0.5" high at 100 yards, and only 1.5" low at 200 yards.
The 100 yard zero is interesting too. It's your classic zero, because that's what most ranges offer for rifles. Interestingly, it is also the peak of the arc. If you zero the AR for 100 yards, it'll never traverse above zero. Whereas most other zero distances will yield a trajectory where the bullet will hit higher than the zero at some yardages. If you think about it, unless all shots are for sure going to be at 100 yards, this says a 100 yard zero isn't that great. Let's assume 1" of play is acceptable. If that's the case, you should select a zero such that a wide spectrum is +/- 1" from zero. A 100 yard zero will have a less generous range of that, because it never goes into the +1" zone. Or in other words, it'll always shoot low unless you're spot on at 100 yards, and have a smaller yardage range where it's hitting within 1" of center. Indeed, this is why the 75 yard zero was pretty attractive to me.
The 90 yard zero was essentially the same as the 100 yard zero.
Conclusion:
The 200 yard zero is close to what I will select. Actually, I'm going to zero the rifle such that it is shooting ~1" high at 100 yards, which is a 60 yard zero. This will have a very flat trajectory out to 200 yards. Theoretically that will hit 0 at 60 and 195 yards. It will be 1" high at 100 yards, and the apex of 1.3" will be reached at 130 yards. At 200 yards it will be 0.3" low, and be within 2 MOA of zero out to 260 yards. Note I said theoretically, because the error of a group from a 16" lightweight Middy is such, that it will be extremely difficult to be certain that this 1.5MOA rifle is zero'd to 1" +/- 10% at 100 yards anyway. In practice, zeroing the rifle, to be a "lil high, maybe an inch" at 100 yards.
Additional commentary:
If serious about indoor use, I suspect one is better served with a laser sight mounted closer to the bore axis than a heads-up holographic sight. But considering most indoor encounters, I'm guessing shooting 3" low really isn't a big deal.