All firearms have a bore line/sight line discrepancy - often referred to as “mechanical offset” - and if you are using them for self defense, you should understand that.
My stock Glocks with standard height sights it is just a bit over .5”, on the Glocks with suppressor height sights and mini RDS mounted, it’s a bit higher still.
•••Again, the below commentary is for red dot shooting with “combat” accuracy standards - not precision bullseye or “sniping”•••
Barms, It ain’t a mindfuck unless you think about it the way you are thinking about it. The bullet doesn’t rise. Due to the angle of the barrel in relation to the sights, it has a trajectory - a flight path that will intersect your line of sight two times: once on the way up, and once on the way down. That’s the simplest way to think about it.
Except for those two times, the bullet will never, ever, ever, hit your exact point of aim. It will always be a little high or a little low.
You need to know the distance for both of your intersection points, your maximum deviation over the line of sight and at what distance you will encounter it, and how fast it drops after the second intersection. This last part lets you figure holdovers for longer ranges - for example, I know with my 50 yard zero that to hit the chest at 300 yards, I need to aim for the neck, at 400 yards, above the eyes.
With an appropriate and useful zero distance, your maximum deviations should be minimal at the ranges you are most likely to encounter. This allows you to not have to think about changing your point of aim for as long as possible.
This is why I like the 50 yard zero. If I do my part, I will never be more than 1.75” high to 1” low from 25-225 yards*. For all intents and purposes - that is point of aim for social engagements.
From 0-25 yards you need to account for mechanical offset regardless of your zero - if you are taking a high percentage shot.
The offset will alway be equal to the distance between the center of your bore and your aiming point at 0 yards and decrease logarithmically from there until you reach your first intersection point or zero distance.
If I need to take a precision shot inside 25 yards, with my preferred 50 yard zero, I know I have to place my dot about 2.5” above my intended point of impact if I am 7 yards and in. I place the dot about 2” high from 15 yards to 7, and about 1.5” from 15 yards to 25 yards and about an inch at 25 yards.
Keeping these numbers in mind, I can estimate anywhere in between if I can gauge my distance to target appropriately. For example - If I am closer to 25 yards, my holdover is closer to 1”, if I am closer to 15 yards away, I hold closer to 1.5” high at 20 yards, I’ll split the difference and try to put my dot 1.25” above my intended point of impact.
I also know that I find a 1” group at 25 acceptable so that’s why I only care about offset inside 25 yards. At around 130 yards I will have the maximum height over line of sight at around 1.5”ish. I am fine with a 4” group there, so I don’t change my aiming point.
So, say my target is an 8” circle. If I aim dead center of it, I will never be more than:
• 2.5”low of center at 0 yards,
• Dead in at 50yards
• 1.5” high of center at 130 yards
• Dead on at 200ish yards
• 2” low of center at 250 yards.
These are all well inside my acceptable hit area of an 8” circle considering a 4” radius.