Rifle scope zeroing question...

[ohbythebay]

I have a question because something I watched (youtube) boggled me but may or may not be correct.

I know how to zero a scope (or so I thought) - Gun rest, 3 to 5 shot group, adjust scope, try again, etc. Basic setup. Not really rocket science. But I have only been able to do this at 25yards as I have not been outdoors to check 50 and 100.

Okay, so I watched a video about sighting a scope on an AR-15 and the guy said at 25 yards, what you want to do is sight to that while your cross hairs are dead on target, the shots should be 2" below the target. His explanation is that if you don't do that and instead get the shots and the cross hairs dead on, your shots will be HIGH at 50 and 100 yards. So there is a bullet rise.

Is this true ? Thanks.

 

[CarJunkieLS1]

OP that seems possible however I can't confirm that. You could look at the trajectory tables from federals website And that may give a decent idea.

I can tell that I sighted a bolt action .22 magnum in a few years ago and it was dead center at 25 yards and then at 50 yards it was almost 5 inches high.

 

[ohbythebay]

Hmmm...

I have to get out of my 25 yard range deadlock. I am meeting an FFL Saturday who does completion shooting, very experienced and well known, etc...He may be able to point me to an outdoor spot where I can get this done.

 

[Art Eatman]

Yeah, pretty close. The deal is the height of the scope's centerline above the bore. Higher on an AR than the typical bolt-action.

Typical bolt action, dead on at 25 is maybe three inches high at 100. With the irons on my AR, I started at two inches low at 25 and was on the paper okay at 100 (I don't remember the specifics).

With a flat-top AR, maybe one inch low at 25 for starters...

 

[The_Armed_Therapist]

For your rifle, there will always be two "ZEROs." That's because the bullet's trajectory is an arch. At some point, the bullet with "arch" to the level of the scope, keep "rising", and come back down to that level again.

If your first ZERO on your AR is at 25 yards, then it will be a while before it gets back down to zero again--well past 100 yards--I THINK around 315 yards. That means that the height of the arch would be around 200-225, which would put your shots at that distance, with that zero, many inches high (6 or 7). They will also be high (but not as high as 200) at 100 yards (4 or 5).

You need to figure out where you want to be dead-on with your shots, figure out the trajectory, and then adjust your sighting in accordingly. Personally, I like a 50-yard zero for my AR. That way, you're only an inch or two high at 100-150 yards at the arch's peak, and your reach your second zero around 200 yards.

 

[jrdolall]

Dead on at 25 yards will get you "on paper" at 100 yards with every center fire rifle I have tried. I have never had to do this with an AR or any other .223 rifle but it should hold true. As Art said a lot of it has to do with the height of the scope rather than the trajectory of the bullet so every setup is different. Trajectory certainly comes into play as well.
One thing to remember is that, at 25 yards, you need to be DEAD ON with regards to windage. Being just slightly off at 25 can be huge at 100 yards.

 

[DPris]

Yes, there can be quite a bit of bullet rise.

Depends on what & where you want to be able to hit & at what distances.

Barrel length, sight type, sight height, bullet weight, bullet velocity, ALL affect point of impact at certain distances.

There is absolutely no uniform "25-yard zero" that holds the same through all ARs & all bullet weights.

I've done tests that resulted in point of impacts running several inches high at both 100 & 200 yards, using a dead-on 25-yard zero.

Don't bother quoting how the military does it.
Don't bother saying "Yeah, but it drops back down at 300 yards."

If all you want is a hit somewhere on a human torso at 100 & 200 yards, a 25-yard zero can do it.

If you want to hit a coyote or a rabbit with precision at those distances, you zero YOUR gun with YOUR sights and YOUR load at the longer distance.

I've gotten up to 13 inches high at 200 yards with a 25-yard zero.
Surprisingly high at 100.

Four different ARs with four different sighting systems, two barrel lengths, two different twist rates, four different bullet weights, different velocities between barrel lengths, all shot to differing elevations at 100 & 200 yards.

If you want precision shooting at longer distances, you zero at longer distances.
If you want ballpark results, you zero at 25 yards.

If you can't go longer, 2 inches low at 25 is better than dead on at 25, but you still won't know for sure what you'll be doing farther out.
Denis

 

[d2wing]

As Art said, it depends on sight height and bullet trajectory. Gravity acts on a bullet as soon as it leaves the muzzle so the barrel actually is pointed up so that at some point the aim point intersects the bullet path as the bullet first goes up then descends. Since the bullet is going up, when it leaves the muzzle it will cross the line of sight twice. I hope this hasn't confused you too much.

 

[jim243]

Gun rest, 3 to 5 shot group, adjust scope, try again, etc.

A little wasteful on ammo. If you have a decent rest to use. Fire one shot at 25 yards (yes one) making sure your rifle does not move, adjust your cross hairs to the bullet hole. You are now zeroed to center of the target. Take the rifle to 50 or 100 yards whichever you wish and again fire ONE shot. Make sure your rifle is locked down and not moving and again move your cross hair to the center of the bullet hole on the target, you are now zeroed at that range. (Yes it only takes two shots to do it.)

Jim

 

[jmr40]

The ONLY reason bullets rise is because the muzzle is pointed upward in relation to the breach. If the barrel is parallel to the ground the bullet will start dropping the instant it leaves the muzzle.

But since the bore is lower than the sights the muzzle will always be slightly elevated. AR's have sights MUCH higher above the bore than most rifles, (around 2-2.5"). This exaggerates the bullets arc. With most AR's a 25 yard zero will also be zeroed at around 300 yards and the bullet will be quite a bit high at 100 and 200. Quite possibly not on paper and a foot or more high at around 200 yards. Unless you are planning on doing all your shooting at 300 yards with an AR about 2" low at 25 is about right at 100. But you'll just have to shoot and see.

Most sporting rifles will have the sights, iron or optics, much lower, around 1/2"-1.5". The bore is always lower than the sights and the bullet will travel in an arc. With most of my hunting rifles zeroed at 100 yards they will be around 1/2" low between 25-50 yards. Perfectly zeroed at 100 and slightly high at around 120 or so before the bullet reaches its apex and starts down again. I'm 2-3" low at 200 yards and 7-9" low at 300. But every rifle and load is different.

FWIW shooting 3-5 shot groups to zero a rifle is a waste of time and ammo. I bore sight and fire 1 shot at 50 yards. Measure the distance it is from zero and adjust the scope. This will put me within 2-3" of where I want it to be at 100. Shot #2 is at 100 yards. I then repeat and shot #3 is always exactly where I want it. I then move to longer ranges to start 3-5 shot groups and fine tune if necessary. I've never needed more than 3 shots to be perfectly zeroed at 100 yards.

 

[ohbythebay]

YOu guys are awesome

VERY helpful information...Thanks !

I may be able to hit the range with 50 and 100 yards on Friday...

Again my thanks...

 

[ohbythebay]

BTW...my scope

This is the one I have

NcStar 3-9X 42mm Mark III Tactical Series Rifle Scope

http://www.amazon.com/gp/product/B00F5V5VJY/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1

 

[Doogledog]

Here's a target that worls pretty well if you can only start at about 25yds. I cant find the link that goes with it but it's fairly self explanitory.
View attachment 198555
Be sure to shoot at farther distances as soon as you can.
You should aim at the red center target and the impact should be center of the gray.

 

[jim243]

Not that much different than one of these, plan on spending a good amount of time on the 50 yard range. Before moving up to the 100 yard range.

Jim

 

[R H Clark]

It depends on how high the scope is above the bore axis. I would definitely sight in low at 25 yards with an AR. Dead on at 25 yards will likely be 6"-8" high at 100 yards depending on your mounts and bullet weight.

 

[ohbythebay]

Doogle dog...

Awesome target....THANKS...

 

[MistWolf]

I watched a video about sighting a scope on an AR-15 and the guy said at 25 yards, what you want to do is sight to that while your cross hairs are dead on target, the shots should be 2" below the target

The higher your line of sight is from the bore, the greater the offset between Point Of Aim and Point of Impact is at the closer ranges. The height over bore for AR sights is generally enough that POI should be roughly 2 inches low at 25 for a 100 yard zero. For hunting rifles with a much lower height over bore, the offset will be much less at short range.

If an AR is zeroed at 100 yards, the bullet will generally only touch the line of sight at one point, which would be of course, at 100 yards

 

[ohbythebay]

One more question

Mounting. Should it be far back on the rail, forward, mid...is it a comfort thing based on your eyes (try the best position) or is there a norm...

 

[witchhunter]

When you mount a scope, leave it a little loose in the rings, close your eyes and shoulder it. open you eyes, move it back or forward until it is clear edge to edge.

 

[1911 guy]

"Bullets rise" and having two zeros are unrelated. You will have two zero points even if your muzzle is parallel to the ground with NO upward trajectory.

Your scope sits above the centerline of the bore. It is a straight line between the scope and the target. The bullet is subject to gravity, unlike the light that allows you to see through the scope.

Drawing an exaggerated example on a napkin, because a full size example would be 300 or more feet long, you'll see that the straight line crosses the arc twice. Height over bore exaggerates the distance between the two crossings.

"bullet rise", is a myth. A bullet only increases its elevation from ground level if the muzzle is pointed in an upward angle. The myth persists because people misunderstand the crossing of the two paths, one of light seen through the scope or sights and the arcing bullet path.

But yes, the height over bore is more on an AR type than a "standard" rifle, bolt action for example. This makes the distance between the two intersections of paths. Which means the standard dead on or one inch low at 25 means not being nearly so close to zero as otherwise.

 

[vaupet]

On the Lapua pages, you can find data as to know your trajectory

It clearly showes how bullets fly. here is an example for 6,5x55

http://www.lapua.com/en/products/sport-shooting/centerfire-rifle/18#tables

If you own a smartphone, there are very nice balistic calculators (I use Strelok)

have fun

 

[ohbythebay]

Okay, thought I had it

Then 1911 guy confused me again...lol

Let me see if I get this because at first I did not...

There is no bullet rise. In fact, I know over distance (long) the bullet starts to drop.

Because the scope is over the barrel by some distance (say 1 inch), it does in fact have an angle to spot the same point on a target thus, if you were to at say 25 yards...when you shoot 100 yards, the bullet will have SEEMED to rise above the center point when in fact, the scope is pointing at the point (downward) at the 25 yard mark. The bullet didn't rise. It was the angle of the scope.

My drawing

 

[R H Clark]

(I think) your scope line should be slightly angled up, your barrel slightly angled up, and the path of the bullet, an arc that increases at distance. The bullet will actually start to drop the instant it leaves the barrel (logically) since it has no wings to provide lift.

Since the bullet is dropping, your scope line, is to aim that bullet at a slight angle up, so that the arc of the bullet and the scope sight intersect at a given point. The higher the scope is above the bore the greater the angle of intersection. So dead on at 25 yards on a scope mounted much higher, has caused you to increase the angle the rifle is angled up for the bore and scope sight to intersect, thus increasing the point of impact at 100 yards from what it would be with a lower mounted scope.

This is just my reasoning and thinking, I could be wrong, but a higher mounted scope on an AR will definitely hit higher at 100 yards when sighted at 25, that a more traditional rifle with lower mounted scope.

 

[Art Eatman]

A similar picture:

 

[wally]

Art's drawing shows the situation correctly.

The scope is confusing you, think of using iron sights, the front sight is the "pivot" point your eye focuses on and the rear is moved up/down, left/right to aim. The scope does the same thing but you are aligning the target image with the reticle inside the scope, the magnification of the target image aids accurate alignment.