Some will tell you setting your zero is very dependent on knowing how far away your target will appear in your optic. This may be true for professional snipers, but I will argue for most defensive applications, it does not matter which zero you choose for a 25 yard target.
Don't get me wrong, knowing your target distance is important, but most defensive encounters will occur under 50 yards, and most likely within 25. The maximum range of a 55 grain, 5.56mm cartridge is around 600 yards. Defensively speaking, I don't really care about targets 1/3 mile away, I just want to be accurate inside 100 yards.
SIGHT OFFSET
AR optics are offset about a 2.8" from the center of your barrel to the center of the optic. In order to close that gap between the optic (POA or Point of Aim) and the bullet path (POI or Point of Impact), what we call the Zero, the bullet must be angled upward as it leaves the muzzle of your gun. In the case of a red dot optic, you do this by lowering the dot, thereby raising the barrel of the gun. But we are talking just millimeters here. Not inches.
If you align your optic parallel with the barrel, it will never be closer to your POA than 2.8”. And at distance, it will be much worse.
Most shooters want to close this gap so their POI is generally close to the POA at most distances they plan to shoot. How quickly your POI reaches your POA is determined by the distance of your Zero, or how far away it is set from the muzzle of your gun. Here are some examples of different zeros:
25 yard Zero - The bullet must rise approx. 2.8 inches very quickly to close the POA and POI gap. With this zero, you will hit a target 50 yards away 2.8” higher than POA, a target 100 yards away, nearly 7” higher! This is really not a good zero for your AR. It essentially disables it from any degree of accuracy beyond 50 yards.
50 yard Zero - The bullet rises 2x slower than a 25 yard zero. At 25 yards, POI will be about 1.5” lower than POA. At 100 yards, POI will be about 1.5” higher. This is an excellent zero because it only has a 3” variability between 25 and 100 yards.
100 yard Zero - The bullet rises 4x slower than the 25 yard zero. At 25 yards, POI will be about 2” lower than POA, at 50 yards, 3/4” lower. At 100 years, zero. This is also an excellent zero for your AR but as I will explain, difficult to dial-in.
Many military specialists like the 36 yard zero for close quarters engagements and medium distance targets. But they also won’t argue the 100 yard zero actually has much less variability between 25 and 50 yards. So why wouldn’t you go with a 100 yard zero?
It is difficult for many shooters to be accurate at 100 yards. It requires a lot of skill and patience to hit targets at that distance. When I train shooters at 100 yards, there are so many things that can go wrong. Just ask a Marine. They train with iron sights at 200 yards. And they do this for days until they have perfected their aim. We don’t have that kind of time or money for bullets.
A 50 yard zero is much easier to dial-in. Far fewer things can go wrong when shooting a target at that range. And most ranges have 50 yard targets. Mine has up to 200 yards, which is great for distance, but not for defensive shooting.
And while POI variability with a 100 yard zero may be optimal, with a 50 yard zero, if my target is within 25 yards, as is most likely the case in a defensive situation, my POI will be a little low. At 75-100 yards, a little high. Beyond that, you aren’t really in a defensive situation any longer.