Scope differences at 100 and 200 yards left to right

Dan,
It is really too bad that you went through the trouble of drawing a picture, because you are still wrong. At least part of the confusion might be that you have keyed into the tiny bit of horizontal offset and dismissed it. Actually that can be dismissed, because it is not the problem. The problem IS that the barrel is no longer aligned with the vertical axis of the scope. You now have 2 components to work with now horizontal and vertical.
When you sight in a rifle you align the line of sight to intersect the bullet path at one distance, and the pull of gravity causes the bullet path to intersect the sight-line at a second point. This is basic stuff, everyone knows all about this. If the bullet was fired in a gravity free enviroment the bullet would intersect the line of sight at only one point, then continue on a divergent path getting farther and farther and from the line of sight because there is no gravity to drag it back. Now since nobody is shooting their rifles in space(Yet!)you might wonder what any of this has to do with anything.I'll tie that together in a second. Since you conceed that a canted rifle will shoot to that side, you also realize that at the zero range the scope is adjusted to intersect the horizontal vecter as well.Right? The problem is that now, after crossing the sight-line from left to right, the bullet will continue to travel to the right, diverging from the line of sight horizontally. Much like the bullet fired in space it will continue to deviate forever, or at least until gravity pulls it to the ground and it stops. The bullet cannot follow the vertical axis of the reticle when it is misaligned horizontally.For your picture to be correct the bullet holes would form a curve down and right, with the down component excellerateing as velocity slows, and the horizontal drift remaining consistant since it is effected by neither velocity or gravity.

It is good to see that you are civic minded, I hope that your club members and all the kids have a ball at the carnival. /ubbthreads/images/graemlins/grinning-smiley-003.gif
Dogleg
 
"When you sight in a rifle you align the line of sight to intersect the bullet path at one distance, and the pull of gravity causes the bullet path to intersect the sight-line at a second point."

Are you saying that, for instance, our military snipers who must be able to engage targets from 100 to beyond 1000 yards must dial in a different windage setting for every range they plan on firing on?

Dan
 
have a freind that has a rifle that if you move the objective lines in the front of the scope it will change from dead on at one hundred to 5 1/2 ins high and a little to the left at 200yd works fine if you do not move nothing
 
bluecat,

That sounds like a bad scope (loose adjustable objective lens if I understand your explanation correctly).


Dogleg,

I don't know how else to illustrate what I'm trying to say on the issue of rifle cant versus scope cant, but I'll give it another try... /ubbthreads/images/graemlins/blush.gif

Imagine that the stock and receiver does not exist. All there is is a barrel with a scope mounted on top of it.

What you contend is that the barrel must be at DEAD 6 o'clock underneath the scope or else the shots won't fall along the vertical crosshair.

What I'm saying is that you can have a slightly canted scope, with the barrel underneath at, say, 5:30 and so long as the scope is held level, the shots will still fall parallel with the vertical crosshair.

Here's another picture! /ubbthreads/images/graemlins/laugh.gif

scopepicture.jpg


In the top image, there is the ideal situation where the scope's vertical crosshair perfectly disects the fall of the shots.

In the second image, the scope is mounted with a slight cant, but since the scope is being held level, this means that the rifle bore is off by a bit underneath it. It's at 5:30 rather than 6 o'clock. Note how the shots fall just slightly to the right of the vertical crosshair. Groups forming downrange would probably never indicate that the 5:30 rifle cant even existed, as these shots would only be off to the right of the vertical crosshair a tiny fraction of an inch.

You mentioned a rifle laying sideways earlier...

If you layed the rifle on its side (and mounted the scope upright, with the elevation turret up top),

sidewaysscope.jpg


...this would of course put the scope about 1.5 inches to the left of the bore. If you zeroed the scope for the shots to fall dead on at 100 yards then yes, you would have an angular relationship with bullet path and line of sight. You'd only be zeroed for 100 yards. From the rifle to the target, you'd begin with almost 1.5 inches of error, slowly correcting until you got to 100 yards, then beyond 100 yards your shots would deviate farther and farther from the windage zero you had at 100 yards. I think we both agree there.

However, if, as is shown in the drawing immediately above, you were to take into account that 1.5 inch difference with the "sideways rifle," and you dialed the scope so that the shots fell 1.5 inches to the right of the crosshair intersection at 100 yards, these shots would stay only 1.5 inches right of the line of sight all the way downrange (wind factors and such notwithstanding).

So, with the slightly canted scope, you're not going to be off 1.5 inches all the way downrange. It'll be more like 1/8" or so--and wouldn't likely even be noticed in a 1 MOA group size.

Guys? Am I making sense here?

I've seen articles written by Wayne van Zwoll and Steve Comus and both of these guys have exhibited a misunderstanding of this issue. They have both contended in print that a canted scope cannot be accurate at more than one range. Sadly, they're both wrong. I mention this only to indicate to all of you that there are guys in pretty respectable places that don't fully understand this issue--so don't feel foolish if you don't immediately "get it." Shortly after reading the Steve Comus article a few years ago I mentioned this canted scope issue in a circle of long range tactical shooters, thinking I was going to teach them something. But I got my legs striped! /ubbthreads/images/graemlins/shocked.gif One instructor by the name of Mike Miller (some of you may know of him) put me straight about it. He pointed out that a level scope was the thing--not so much a perfectly level rifle underneath it. I re-thought the issue and realized he was right. But of course he was, he does this stuff for a living! So... I believe I have explained it as well as I possibly can, so you folks can decide what to think. I base what I've written here not only on quite a bit of in depth thought on the matter, but also on having seen (since my enlightening encounter with Mr.Miller) rifles which had obviously canted scopes, zeroed for windage at 1000 yards, still be pretty much on the money (windage-wise) at closer ranges--even 100 yards, so long as the scope was held level. The issue is truly as simple as a level scope. But it's pretty easy to overthink it...

Dan
 

Are you saying that, for instance, our military snipers who must be able to engage targets from 100 to beyond 1000 yards must dial in a different windage setting for every range they plan on firing on?

Dan




No, I'm saying that if they mounted their scopes canted they would have to.
Dogleg
 
You guys are both right but not explaining it very well.

It does not matter how much you cant the rifle. However the crosshairs must be vertical and centered directly over the barrel, if you want to avoid a windage change every time you change elevation. That is difficult to do with standard mounts because they normally center the scope over the barrel only when the rifle is not canted.

ISU competition rifles have used sights that are tilted so the sight is vertical over the barrel when the rifle is canted for at least 20 years. You buy a mount with the amount of cant you use and your sight is vertical to the world and vertical to the bore.

Jack
 
David Tubbs, If any one knows who that is shoots with a radical cant on his rifles. He has to dial in windage settings and elevation settings for each distance he shoots. If the scope is canted when mounted on the rifle, the shooters eyes will hold the scope level. which in turn sets the bore of the rifle at a cant. it will only be dead on at one distance. That being the distance it is sighted in for. as the distance increases the bullet will deviate in a windage type situation.
 
Dan,
Your model only works if the line of sight and bore-line were paralell, which they are not.I'll give you an example.It is widely accepted that a typical sighted-in big game hunting rifle with scope mounted 1 1/2 inches above line of bore will first cross the line of sight at about 25 yards. I'm sure that you will agree that that is approximatley correct, right? Close enough for government work as they say.
Ignoring for the moment the tiny bit of bullet drop over 25 yards (900 inches) and assuming that the line of sight is level; then the angle that the barrel HAS to be at to make this happen will be:
acos(-1.5 divided by 900)= 90.0955 degrees. Still with me?

Since the difference between between 90 degrees and 90.0955 is .0955 this is the angle of divergence. (AD)

Now since course deviation(CD) is Sine (AD) X course length (The next 75 yards)
or Sine .0955 X 2700 inches = 4.5 inches

That is the distance that the line of bore and line of sight will have separated at 100 yards.

So if the rifle was laying on it's right side the impact would 1.5 inches left at the muzzle, dead on at 25, and 4.5 inches right and low at 100 yards.At 200 it would be 10.5 inches right and real low because the bullets have no upward angle to help compensate gravitional pull.

Now a canted rifle is neither straight up or straight sideways, but a portion of both, agreed? The amount of horizontal deviation at 200 yards due to rifle cant is Sine (Cant angle)X 10.5" in this case. Let's use 10 degrees for this example. Sine 10 x 10.5 equals 1.8233 inches to the side and you have just missed your prairie dog.

I really don't think that I can explain it any simpler than that, that should suffice for some-one that wanted to learn. It is definitely much easier to remember that straight mounting is important than to remember the math. /ubbthreads/images/graemlins/laugh.gif


climberguy;
This thread has moved away from it's original topic, (Your rifle) and for that you have my regrets. You may also want to check to see if your scope tracks through power changes, and paralax adjustments but I suppose you would have checked that long ago.
Goodnight gentlemen,
Dogleg
 
Quote:
Jack you were typing while i was. You said it better.



17shooter,
Actually you said it better! Where is David Tubbs when Ya need him??

Jack Roberts,
What you say about properly canted sights on a canted rifle eliminating windage changes while making elevation corrections is of course correct. It would allow you to move your POI straight up or down on the target.It only applies at one distance at a time though. Change the distance and windage adjustments will still have to be made. 2 straight lines can only intersect at one point.
Does anyone else have a headache? /ubbthreads/images/graemlins/grinning-smiley-006.gif
Dogleg
 
Ok I tried tc223's suggestion with looking in a mirror with my rifle. It did seem to be a little canted based on that. I looked at my other rifle in the same caliber, .223, and it seemed to be oriented correctly in the mirror and I dont have this problem with it. It has a 3x10 scorpe so it is more clear on the lowest power, the rifle I am having problems with has a 6x18 nikon and it is harder to get the crosshairs clear at that magnification. I resighted at 100 yards and will let you all know what happens at 200 when I get to the range. Thanks
 
guys... guys... guys... /ubbthreads/images/graemlins/laugh.gif

It's bullet path, bullet path, bullet path--not the barrel that you're aligning the scope with.

For those of you who believe that a canted rifle (bore not directly at 6 o'clock) will cause windage diversions, please explain this:

You have a perfectly mounted, level scope. You dial it in for a 200 yard zero and shoot a very tight group which falls perfectly centered on the bullseye.

Then you switch to another brand of ammo, or another handload.

You notice that your shots are no longer centered for windage at 200 yards.

Why?

Because barrels vibrate and whip and they can therefore throw bullets pretty much anywhere in a conical pattern eminating from the muzzle.

You find that with your new load, you have to not only correct for elevation at 200 yards, but for some odd reason /ubbthreads/images/graemlins/tongue.gif you have to correct for windage as well.

bulletpath.jpg


The above is a top view, looking down at the rifle and scope, and bullet path(s). The blue area would represent the scope's range of WINDAGE alignment; in other words it can be dialed to windage zero anywhere in that blue zone. The orange lines represent the various paths different load recipes might cause the bullets to fly. You can dial the scope to align with any of these paths.

All that really matters is that the bullets be released relatively close to the vertical crosshair and all will be well.

When the barrel releases the bullet, the bullet goes up, then it comes down. It does this regardless of where the barrel is in relation to the scope. All you are doing with the scope windage adjustments is aligning the erector to be parallel with that bullet's path. The key word here is PARALLEL, and once the scope's windage is set to be parallel with the bullet's path, you will not have to change windage for various ranges--it'll stay the same--even if the barrel is at 5:30 or 6:30, or even 5 o'clock or 7 o'clock underneath the scope.

That's just the way it is guys. I have no idea what David Tubb would say, but if he differed with me here I would be very surprised. (And yes, I would argue with him too if he said otherwise! /ubbthreads/images/graemlins/laugh.gif )

The problem that some of us may be having in understanding this is we are assuming that bullets eminate in a straight line from the bore of the barrel--which they do not. Some bores are not even drilled straight through the barrel, so you could have a scope centered "perfectly" over the barrel and still have a "canted bore," and still shoot just fine.

Too, I think some of us here are basing our positions on what we've read, what we've been told elsewhere, and what we've surmised. I shoot to distances beyond 1000 yards fairly regularly, and have learned that it is the level scope that is key.

I have a ScopLevel on a Bushnell Elite scope. The level is mounted to the tube of the scope, and I move that scope from rifle to rifle as I test different guns and different loads. It's currently on my recently rebarreled Remington 788, and it is not perfectly straight with the receiver. I noticed this when I put the rifle on a bench at our rifle range. However, since I began testing that rifle, I have hit varmints at 200 yards, 540 yards, and 755 yards by making elevation changes only. I did not make windage changes for these shots because the wind wasn't at issue. A couple weeks ago, again, without touching the windage turret, I dialed in my elevation for 1050 yards and shot at this target:

1062yds.jpg


While the group has moved a tiny bit to the right, that is to be expected at such long ranges as the bullet's spin causes it to move slightly aside.

Dan
 
I'm gonna try to put this argument into simpler terms, please respond in kind.
When you mount and zero your scope, you establish a line with it. This line goes from your eye to the target.
When you aim the gun (and therefore the barrel) you establish a line (let's ignore gravity for now) from the breech to the target. These two lines intersect. So, ignoring gravity, you can think of this as a very flat "X", or maybe ">
 
JimT,
/ubbthreads/images/graemlins/ooo.gif, /ubbthreads/images/graemlins/ooo.gif Exactly! You have a simpler way of explaining it, but then again I already know what you mean. Dan will not be easily convinced, because he is still trying to make the two lines parallel. Until he grasps the concept that the line of flight has to cross the line of sight on two planes on a canted rifle, and only on one with a straight one he isn't going to get it, perhaps never will. I really hope that you can do a better job of explaining than me, I tried. My concern now is for others that read this thread and may get the wrong idea.Your bow example is excellent, long range black powder is another.
Dogleg
 
Jim T, you're right with one exception. You mention: "When you mount and zero your scope, you establish a line with it. This line goes from your eye to the target.
When you aim the gun (and therefore the barrel) you establish a line (let's ignore gravity for now) from the breech to the target."

Here's the thing. While you do establish a line with the barrel of the rifle, that line is not really important. The line of the barrel and the bullet path will not be the same. If this was true, you wouldn't have to windage zero for different loads, you could just put any ammo in the gun and it would all go right down that same line that the barrel points. But we know this isn't true; bullets are released in different directions by the barrel, and we must dial the scope to follow the bullet path, not the barrel.

Forgive me for sounding like a broken record, guys, but we cannot logically assert that the barrel's line of sight is one and the same with bullet's path--it never is, because barrels bend and whip, sending the bullets off in different directions depending on the load recipe.

Jim, I do like the way you're thinking about this, and I believe it is possible to bring you around to what I'm trying to say here. /ubbthreads/images/graemlins/laugh.gif

Draw the ">
 
Actually, that makes perfect sense.
I see now why you and dogleg couldn't agree on this. Dogleg, like me, was assuming that you want the bullet to impact at point of aim. You're talking about canting the rifle and shooting to the side of point of aim. Assuming that you can get a straight path out of the bullet, then yes, there would be zero windage error. BUT, you have to compensate for your fixed windage offset.
You are setting the rifle up to shoot canted from the beginning, understanding that there will be an offset all the way down.
Dogleg and I are starting with a rifle that is sighted in uncanted, then turned. In the situation we are discussing, we are correct. In the situation you are discussing, you are correct.
How's that for a happy ending? /ubbthreads/images/graemlins/grin.gif

Edit to clarify it one last time:
You use parallel bullet (bullet or bore is really just semantics in this situation) and sight lines.
Dogleg and I are talking about converging bullet and sight lines.

Quote:
Now. Put 1/16", or 1/8", or even 1/4" of space between these parallel sheets. Can you see now that it is possible to have the scope's sight line and the bullet's path not be perfectly right on top of each other, but still be parallel within 1/4" (or whatever) of each other, all the way down the range? A canted rifle underneath a level scope might shoot along Sheet B, even though the scope is sighting along Sheet A.

Does this make sense now?

 
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Climber guy said.


Quote:
My scope is dead on at 100 yards but it is 2 inches right at 200 yards on a dead calm day. Could this be due to the mounting? Any help would be appreciated.



Dan Said
Quote:
You have a perfectly mounted, level scope. You dial it in for a 200 yard zero and shoot a very tight group which falls perfectly centered on the bullseye.

Then you switch to another brand of ammo, or another handload.

You notice that your shots are no longer centered for windage at 200 yards.




Dan all that you have said is correct, I think. It is getting to be too much for the limited neuronic activity in my brain to handle. The problem is that you are talking about apples and he asked about oranges. He didnt change ammo, He didnt change scope settings. he simple shot at 100 yds, moved back to 200 yds and shot again. the bullets group moved in a windage deviation on a calm day. this can only be explained by rifle cant.
If the bullet leaves the muzzle in a rifle that the barrel is not 6 oclock below the cross hairs intersection, it will deviate in a windage scenario and continue to do so for as long as it flies.

If you dont not believe me, take one of your rifles , a riflethat I am absolutely assured by reading your post for several months is set up correctly, to the range. shoot it with a 10 degree cant to the left. then shoot it with a ten degree cant to the right. at 100 yds. now move back to the 200 yd target and do the same. then report back to us that it didnt change the windage POI.

They put scope level bubbles in the bottom inside of a Springfield armory sniper scope for a reason.

If the scope is mounted with a cant and the verticle wire is not disecting the center of the bore. It can not be expected to shoot along entire path of the bullet without windage deviation.

As I stated before, in this same thread, If the scope is canted the shooter will level the scope on the target, which in turn moves the bore from under the scope. The bullet is no longer rising and falling in straight line to the target. It has a built in windage deviation. The bullet should cross the line of sight two times on its way to the target, once up, and once down. If the rifle is canted it will pass the line of sight only once and get farther away from that point as it continues its flight.
 
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JimT,

You wrote:

You're talking about canting the rifle and shooting to the side of point of aim.

That is exactly right, Jim. You do have a good way with explanations!

In truth, we are almost always shooting to one side or the other of point of aim. (Otherwise, we'd be shooting bugholes!) /ubbthreads/images/graemlins/laugh.gif

So what you're saying is right; in the canted rifle/level scope scenario, the bullet path will be essentially parallel with point of aim--and in your words the bullet will shoot to the side of the point of aim.

At 300 yards, with even a 1/2 MOA group (about 1.5 inches) a line of sight which was offset 1/4" would have the center of the group moved 1/4 inch to the right, which is of no real consequence. Such a minor amount gets "lost in the noise" as they say. This 1/4 inch of offset would still be 1/4 inch at 1000 yards, and beyond--but by the time you get past 100 to 200 yards, you'll never really notice such a small amount of offset.

This is a good thing, too, because as I mentioned earlier you cannot usually get rid of this offset. When the bullets emerge from a curved (vibrating, whipping) barrel, they're already offset from the line of sight by a small amount (unless you're the lucky one and you have a vertically whipping barrel which is perfectly whipping up and down your perfectly level vertical crosshair!)

A 26" magnum sporter weight barrel whips a lot, and it would not be surprising to see even more than 1/4 inch of offset at 100 yards when the windage was dialed to be parallel with the bullet's path.

Many of you probably own rifles which are dialed in for windage at 300 yards (for instance), and they seem "dead on" for windage at 500, 600, and even farther. But bring it back to 100 yards (where you can shoot smaller groups) and you may see that 1/4 inch or more of windage offset appear on the target--even though the scope is level and centered over the bore. The barrel has "whipped" the bullets onto a line which the scope's line of sight cannot fully converge with; so the small parallel offset is what we must accept.

Thanks for your time and attention here, and your excellent sense of understanding.

Dan
 
Oh and by the way climber guy, welcome to the board. I bet you didn't think this was going to happen when you made that your first post. /ubbthreads/images/graemlins/smiliesmack.gif
 
17shooter,

You're 100 percent correct. When Dogleg mentioned that 12 degrees of cant would do this, I did not dispute that--but I surmised that Climber Guy may have canted left 6 degrees at one range, then right 6 degrees at the other to make up the full 12 degrees. Canting a scope 12 degrees is more than it may sound, and most guys (most, I emphasize) would notice that much cant. Six degrees, I can buy.

We got off on a tangent point about whether a canted rifle with a level scope would shoot straight (which as you correctly point out had nothing to do with Climber Guy's question). /ubbthreads/images/graemlins/blush.gif But it's been an interesting discussion, and my MSpaint skills are some improved. /ubbthreads/images/graemlins/laugh.gif

But again, it is true what you're saying. Canting the scope will cause the shot to fall aside downrange. I just don't see how someone could cant the scope 12 degrees and not realize it, hence my 6 and 6 notion...

Dan
 


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