Hi there. Just installed a Primary Arms Red Dot on M&P, I know its not high end (like a Trijicon), but it will do the job until I can save for a better one. Took it to the range and I noticed at 10-12 yards, its spot on. At 13-17 yards, I have to aim approximately an inch lower than where the dot is. At 20-25 yards, an inch and half lower to nail the target. Is it my eyes or the bullet?

Welcome to the Forum.

That red dot you're looking at is about 2.5 inches above the bore of the barrel. That's typical of an AR platform.

Think about this for a minute. If your zero is at 12 yards then the muzzle has to be pointed up in relation to line of sight to make up for 2.5 inches in 12 yards. So where do you think that bullet is going to be at 24 yards? About 2.5 inches high in relation to line of sight (slightly less than 2.5 high inches due to gravity but that's not the issue were trying to illustrate on this). This is why you're having to aim lower at 13-17 and 20-25 yards.

Here is simplified view of what's going on. Where the blue and green intersect is your 12 yard zero. Notice the blue line (bullet) begins to travel above line of sight beyond 12 yards.



Most folks zero an AR at 50 or 100 yards. The bullet trajectory looks something like this with a 50 yard zero. Notice the bullet crosses line of sight again at around 200 yards. This illustration includes the effects of gravity on bullet trajectory.