AR15 Optics Question


PDA






IndianaUser
June 1, 2010, 02:35 AM
I purchased a (very overpriced) Osprey tactical scope awhile back and just got around to zeroing it in a few weeks ago. After I zeroed in the scope at 25 yards and was attempting 50 yards, I had a jam that rendered the rifle unusable for the rest of the day.

Goal: Zero in at 100 yards

Step One: use bore sight to get on the paper at 25 yards - Check.

Step Two: zero in at 25 yards. Check.

Also worth noting that at 25 yards the laser and ammunition were hitting exactly the same place. Assumption: the laser is pretty much exactly where the bullet would hit.

Step Three: increase to 50 yards. Not even on the paper. I can't use my laser as it doesn't show up at 50 yards in daylight. I use my BDC and/or rangefinder to compensate for any drop and still can't get it on the paper. I was going from the zeroed 25 yards and adjusting all the way out to extremes and could not get on the paper.

Step Four: I put my boresight back in the rifle and notice the laser deviates vastly from the crosshairs as I aim at different objects +/- a few yards from the 25 yard target.

Step Five: I put the boresight in my rifle and aimed through the scope the length of my house (should be about 20 yards). The laser was oriented two crosshairs down from center. If I aim at something about 8 yards away, the laser moves to the very bottom of my field of view. If I aim at something about 6 feet in front of me, the laser is completely below my field of view.

Problem: vertical orientation fluctuates wildly as a product of distance. Horizontal orientation seems constant regardless of distance.

What am I doing wrong?

If you enjoyed reading about "AR15 Optics Question" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
Rail Driver
June 1, 2010, 02:48 AM
Well, my first thought is that with standard 5.56 and a 16" barrel, you're only going to experience about 1"-1.5" drop at 100 yards, at 50 yards it's more like 0.2" drop. You may have been over-compensating for the anticipated bullet drop.

When zeroing a scope, you want to keep in mind that the scope is a fair distance above the center of the bore on an AR... anywhere from 1" to 3" or 4" depending on your rig. At 25 yards, zeroed, your scope is going to be aiming LOW at 50 yards, rather than high and it will be aiming VERY low at 100 yds on a 25 yd zero.

My advice would be to find out how to calculate scope offset based on how far above the bore your scope is mounted and use that information to adjust your scope to 50 or 100 yds after you re-zero it at 25 yds. You should be able to have your rifle dialed in at 100 yds using as few as 3 shots, but no more than 9 shots.

Here's what I do and it works well for me.

1 shot at 25... adjust accordingly to zero the scope to 25 yds (confirmation shots as needed to check scope adjustments), crunch your numbers to figure out how many clicks you need to go UP with your scope to adjust the zero from 25 to 50 yds, fire and check your 50 yd target... adjust accordingly to zero to 50 yds, crunch numbers, adjust scope up to zero at 100 yds, fire. Your rifle SHOULD be zeroed at 100 yds now.

Also, very close range + scope = bad.
Iron sights or red dots are good for close range. Scopes with more than 4x magnification are relatively useless for short range unless you plan to miss.

JJ Hiryuu
June 1, 2010, 02:54 AM
Well, lets see what we can eliminate as a possible cause of the problem.

Did your scope come with any specific instructions on zeroing?

What barrel length and twist rate do you have?

What brand ammo and what bullet weight?

What kind of mounts are you using for the scope?

Does the rifle have irons, if so are they on target?

Is the scope a variable zoom type? Did you try it at 50 without touching the BDC?

M1key
June 1, 2010, 03:01 AM
Once you get the above figured out, set clay pigeons on a berm at 100 yds and have someone spot for you...or do it yourself through the scope. Dial it in until you can hit the clays reliably, then fine-tune on paper.

About vertical orientation: mount your scope as low a possible to the axis of the bore.

halfded
June 1, 2010, 08:06 AM
Sounds like parallax might be the culprit here. Look at a target 25 yards away through the scope. Move JUST YOUR HEAD not the scope. Do the crosshairs move on the target? If so, that's parallax. It'll go away at (usually) 50 yards and greater.

Boresight the gun visually by looking down the barrel at 50 yards, then fire a couple rounds and see if you're on target.

IndianaUser
June 1, 2010, 11:08 AM
JJ

* 16" upper. Unsure of the twist or lining.
* Scope came with instructions. All pretty typical and followed to the T.
* Shooting 55gr Wolf .223 steel cased ammo. Ammo was pretty much dead on where my laser boresight was hitting.
* Rifle has irons and they are on target.
* It is variable magnification. I tried everything without using the BDC as well. I really should have to use this from 25 to 50 yards though.

taliv
June 1, 2010, 11:14 AM
ime, laser boresighters cause more problems than they solve. just take the upper off the lower, rest it on the bench and look through the barrel at the target. you should be able to easily see the target at 50 yrds through the barrel.

IndianaUser
June 1, 2010, 11:22 AM
At the range I put my boresight in several times and it was almost exactly on point at every distance.

IndianaUser
June 1, 2010, 11:25 AM
Illustration. The red dot indicates where the bullet is hitting within my field of view. The 25 yard distance is exact, the others are educated guesses. I am at the intersection of the cross hairs for every shot.


25 Yards
http://img404.imageshack.us/img404/6323/rangefinder25.gif


20 Yards
http://img718.imageshack.us/img718/2954/rangefinder20.gif


8 Yards
http://img708.imageshack.us/img708/530/rangefinder8.gif


5 Feet
http://img441.imageshack.us/img441/1017/rangefinder5ft.gif

taliv
June 1, 2010, 11:42 AM
your illustration is exactly what i would expect. that's not a problem.

i thought you said the problem was you were zeroed at 25 yrds but not on paper at at 50

benEzra
June 1, 2010, 12:15 PM
To be right on at 50 yards, the point of impact will need to be about 1.25 inches lower than the point of aim at 25 yards, and that's assuming your scope is mounted fairly low on a flattop upper. I suspect you may be shooting over the 50-yard target if you are zeroed right on at 25 yards.

Problem: vertical orientation fluctuates wildly as a product of distance. Horizontal orientation seems constant regardless of distance.
This is as it should be, because the bullet starts out about 3 inches below the line of sight (more if you have a high mounted scope) and shouldn't intersect the line of sight until about 50 yards.

IndianaUser
June 1, 2010, 12:27 PM
That is correct. Zeroed at 25 yards (as shown in first picture), but not even on the paper at 50. If we extrapolate this trend, I would think the bullet is probably traveling over the top of the paper at 50 yards.

I'm obviously not very experienced, but this huge swings resulting from minor changes in distance really confuse me. If I've zeroed at 25 yards, the bullet should hit lower in my field of view as I increase distance. The opposite is happening and the swings are extreme.

According the this ballistics chart, there shouldnt really be much of a difference between 8,20,25, and 50 yards. I'm seeing huge swings.

http://img715.imageshack.us/img715/5733/ballisticschart.png

I'm misunderstanding some fundamental concept here, but am not sure what.

Kwanger
June 1, 2010, 12:31 PM
If you are zeroing poa to poi at 25 yards, but not on paper at 100 yards, you will in all probability be shooting over the top of the target.

I zero with the impact very low at 25 yards (6 inches at least), then move out to 100, then 200. Zering magnified scopes at very short ranges, especially scopes with a high axis above bore, is a pointless excercise beyond getting centered for windage and rounds low on the paper.

With scopes hig above the bore, think about it in extreme terms. If your scope was at the top of a building and your barrel on the ground floor and the target was across the street, your scope would have to be pointed downwards very severely. Move the target further out and keep that same setup - your scope will rotate up towards horizontal, but now your barrel will be pointed skywards - this is effectively what is happening when you zero bang on at 25 then move back.

IndianaUser
June 1, 2010, 12:34 PM
Ben - the second part of your comments makes a lot of sense. Thank you.

So if I extrapolate this line of reasoning, the deviation between the crosshairs and point of impact should not deviate more than 3" (or the height of the scope mount) when shooting at very close distances. For example, if the bullet impacts almost immediately after discharge, the bullet impact should be about 3" below the center of the crosshairs. Is this correct?

benEzra
June 1, 2010, 12:54 PM
So if I extrapolate this line of reasoning, the deviation between the crosshairs and point of impact should not deviate more than 3" (or the height of the scope mount) when shooting at very close distances. For example, if the bullet impacts almost immediately after discharge, the bullet impact should be about 3" below the center of the crosshairs. Is this correct?
Exactly. For a sight height of 2.8 inches (which just above the tip of the AR's front sight) and a 100-yard zero, you're looking at a trajectory like this:

0 yards, bullet is 2.8 inches below the line of sight
25 yards, bullet is 1.7 inches below the line of sight
50 yards, bullet is 0.9 inch below the line of sight
75 yards, bullet is 0.3 inch below the line of sight
100 yards, bullet is at the line of sight
125 yards, bullet is at the line of sight
150 yards, bullet is 0.4 inch below the line of sight
175 yards, bullet is 1.2 inches below the line of sight
200 yards, bullet is 2.3 inches below the line of sight
225 yards, bullet is 3.8 inches below the line of sight
250 yards, bullet is 5.9 inches below the line of sight
275 yards, bullet is 8.4 inches below the line of sight
300 yards, bullet is 11.4 inches below the line of sight

For a 50-yard zero instead, which is my own personal preference, it'd be like this:

0 yards, bullet is 2.8 inches below the line of sight
25 yards, bullet is 1.3 inches below the line of sight
50 yards, bullet is right at the line of sight
75 yards, bullet is 1 inch above the line of sight
100 yards, bullet is 1.7 inches above the line of sight
125 yards, bullet is 2.1 inches above the line of sight
150 yards, bullet is 2.1 inches above the line of sight
175 yards, bullet is 1.8 inches above the line of sight
200 yards, bullet is 1.1 inches above the line of sight
225 yards, bullet is right at the line of sight
250 yards, bullet is 1.6 inches below the line of sight
275 yards, bullet is 3.7 inches below the line of sight
300 yards, bullet is 6.3 inches below the line of sight


The 50-yard zero puts you a tad high between 75 and 200, but keeps the bullet closer to the line of sight at 300.


If you zeroed exactly at 25 yards, you'd be 2.7 inches high at 50, 7.4 inches high at 100, 14.2 inches high at 200, and 16.4 inches high at 300, according to my iPhone, and if you dialed in any additional elevation then it would be easy to shoot completely over the target, which may be what happened.


One thing I do when sighting in a new optic is to use the biggest target and/or the biggest cardboard backing that I can find. If your target is 3 or 4 feet tall, it's easier to get the rifle on paper at 50 or 100 than if you are shooting a 12" target. But if you are in paper at 25 yards, adjust the scope so that the bullets are hitting about 1.5 to 2 inches below where you are aiming and that should at least get you on the paper at 50.

Ratdog68
June 1, 2010, 01:25 PM
I've got to agree with benEzra's post. Zero at 50yds, then see how much it "rises" at 100yds, and how close "back to zero" the impact is at 200yds.

M1key
June 1, 2010, 03:07 PM
Maybe a picture will help...

mshootnit
June 2, 2010, 12:43 AM
I have had some confusing days at the range especially with russian optics way above the bore. Here is what helps. Start at about 7 yards. Shoot and adjust to get shots coming in about as low as your optic is high. back up to 25 and try it. Go for about one inch low here. Then go to 50 meters and zero. Now go to 100 then go to 200. Another thing that helps alot is a laser bore sighter.

If you enjoyed reading about "AR15 Optics Question" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!