Explain rifle zero point.

B

BoogieMan

New member
Is it when the upward arc peeks is zero? Or is it when the arc down crosses the muzzle point of aim?
Is the zero point dependant on the rifle or the cartridge?
Should every rifle be zeroed (POA) at the yardage required to hit the arc zero?
My hunting rifles (30-06) are all zeroed at 100yds. What does that mean at 200yds? drop or rise?
 
Let me see if I can 'splain this. Your eye sight is straight (doesn't matter if through a scope or irons). It doesn't curve. It isn't affected by gravity. Your bullet doesn't have that luxury. It starts dropping towards the center of the earth the instant it leaves the barrel. That's just the nature of gravity. In order for a bullet to hit any target in front of the barrel, the barrel itself has to be angled slightly upward. Think of throwing a football. In order to hit the pass receiver, you have to let go of the ball so that it travels upward before it starts traveling downward - a nice big arc.

Putting that into rifle terms, this means that the barrel is always going to be angled upwards in relation to Line-of-sight. Since your line of sight is above the bore of the barrel, this upward angle causes the bullet to cross the line of sight twice - once on the upward side of the arc and once on the downward side of the arc. The first crossing will occur very close to your barrel - usually within the first 20 yards or so (give or take). The 2nd crossing (as the bullet is on its downward arc) is known as ZERO. The distance from the muzzle to that zero mark is what you are adjusting when you move the scope/sight's elevation.

Now, go back to that arc thing again. The faster a bullet is, the less of a curve that arc has for a given zero range. In other words, if you have a relatively fast rifle (like your 30-06) zeroed in at 200 yds and another relatively slow rifle (like a 30-30) also zeroed in at 200 yds, the arc for the slower 30-30 bullet will be significantly higher and more curved than that of the faster 30-06 bullet. The faster the bullet, the less of an arc you have to have to get the bullet out to a given distance.

As to sight-in range, you aren't doing yourself any favors at all by zeroing in a 30-06 at only 100 yds. At that zero, the highest point of the arc will be somewhere around 75 yds and will only be about 1" or so high. At 200 yds, your bullet will drop several inches from your line-of-sight zero point. Instead, changing the zero to 200 yds means that the highest point of the arc will now be somewhere around the 100 yd mark at about 2" high but now 200 yds will be at line-of-sight. The same "several inches down" that you were dealing with earlier is now stretched out to about 250 yds.

I don't have any ballistics tables in front of me so I'm using rounded numbers from memory - but I hope you get the idea.
 
Is it when the upward arc peeks is zero?
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No, but the term might imply that.

If you take a ruler and draw a straight line (line of sight). Then draw an arc over that line, so that the arc intersects the straight line at 2 places. This is the bullets path.

As you can see, the bullet will cross the line of sight up close (depending on caliber and load this might be around 25yds) and it crosses again at your "Zero" distance.

The close distance allows you to "get close" when at the shooting range in order to fine tune it at whatever distance you choose to "zero".


EDIT...Doyle beat me to it and is SPOT on
 
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I think I got it.
My zero is always going to be the second line x arc intersection. Would it be a general rule of thumb that point should move out with faster higher BC bullets.
200yd for a 30-06
25yd for a 45acp
Etc....
 
Zero is the range at which your scope is calibrated to hit a target. That could be anywhere between 15 yards and 1200 yards. (give or take depending on the equipment and a lot further with some unusual equipment) It just all depends on what kind of cranking you do on you knobs.
 
The rifles zero is where when it hits the point you are aiming at. That could be while it is still traveling up, or after it has reached it's apex and is coming back down. I can have rifle sights adjusted so the bullets point of impact is dead on at almost any range.
 
Thank you Sweet Shooter for your graphic. It is said that a picture is worth a thousand words...but in the discussion at hand, it is worth at least ten thousand.
 
terms

Doyle's got it, but While we're at it, let's use some terms.

Line of Sight is pretty self explanatory. Dead flat, like a laser, from your sight system to the target.

There will be two points where your bullet crosses Line of Sight; one where the bullet rises from the muzzle to the "initial intersect" with Line of Sight, and the second where the slug drops back down to the second intersect, which is typically called "the zero".

Middle ordinate,, max ordinate, or Mid Range, is the point where the bullet is at it's highest point on the arc between the two intersects, (II and zero). Mid Range is not halfway, but typically about 2/3 the way to zero.

After the slug passes thru "Zero" it will continue to drop, and when the drop is equal to the "Mid Range" that is typically called the "working range" of the cartridge. I have also heard that distance called "point blank" range, contrary to the common interpretation.

Some comments: Big scope bells, high rings, see thru rings, any high sight system, tends to work against a flat trajectory, though not radically, but enough to bug me, so I avoid such rigs, which have other disadvantages as well.

The old, sage advice for a 100 yd zero on "typical" deer cartridges was +3" at 100. None of my rigs are set up that way these days. I now have access to a 200 yd range, and shoot to "zero" at that distance. When I come back to 100, I find that most of my rifles will be 2-2.5" high, and that suits me fine. That limits my "working range" a bit, but keeps the bullet closer to line of sight at mid range, and at the distances I shoot, that's more important than 50 yds or so of more "point blank" . Before I had 200 yd benches, I zeroed 2" high at 100, and that worked pretty good too, but I was probably coming in a little low at 200 with the slower cartridges. Didn't really matter, as I rarely shoot at a deer that far anyhow.

I do zero modest cartridges "on" at 100. My .44 carbines, the 7.62x39 bolt rifle (and AK) are all on at that distance.
 
After the slug passes thru "Zero" it will continue to drop, and when the drop is equal to the "Mid Range" that is typically called the "working range" of the cartridge. I have also heard that distance called "point blank" range, contrary to the common interpretation.
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The reason most it is called point blank range is that, depending on how your rifle is zeroed, inside that range, you aim for the center of the vitals, your bullet will never be outside the vital area of the target.

It is the same concept of the battlesight zero used by the army on the M16, zero the rifle at 25 meters, any range out to 300 meters, you hold center mass and you will hit.

Example, a .308 150gr bullet @ 2800 fps with a 1.5" sight height, using a 4" vital zone radius, your point blank range zero would be 260 yds, and your max point blank range would be 305 yds.

So if you are hunting, any deer you see inside 305 yards, you put the crosshairs in the center of the chest, and the bullet will hit in the vitals.
 
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