Juggle levels on rifles

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Last Post 26 March 2016

CBA President

John Alexander posted this 21 March 2016

That is supposed to be bubble levels but I know see how to change it. Oh well.

Little tiny levels have been used on target rifles, especially long range target rifles for a long time to avoid canting the rifle or maybe to allow canting it the same amount for each shot. They seem to make sense at least in theory for iron sighted rifles. However, I have always wondered why they might be useful for scopes with a horizontal cross wire that lets you control can't very precisely by alining the cross wire with something horizontal in the field of the scope (top of targets. target frames etc.) which seem intuitive and natural and doesn't require that you divert your eye from the target.

Strangely, i have never heard it discussed why it makes sense to hang a level on the outside of a scope and I'm curious. Thoughts anyone?

John

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CBA President

John Alexander posted this 26 March 2016

Eating humble pie isn't so bad when you get used to it. If I use a sight adjustment to elevate the muzzles that it actually takes to get zero at 1,000 which might be 30 moa or approximately 300 inches instead of the stupid 1 degree used in my example (convergence at 200 yd) my equation (300 inches x sin 1 degree) produces 5.2 inches of deflection similar to LMG's and Mike H's numbers. This is approximately .5 moa.

30 moa would be about the right sight elevation for a 311299 bullet with a MV of 1,800 fps at only 600 yards and much more at 1,000 yards so the deflection for such a cast bullet at a 1,000 yards would be quite a bit more. Note that the midrange height of the bullet doesn't enter the calculation and doesn't need to be known -- just the sight adjustment needed. For folks interested in deflections due to changes in cant in a CBA 100/200 yard match using the 299 bullet at 1,800 fps with the scope 2” above the bore, and the method above one degree cant would deflect the bullet 0.06” at 100 yards and 0.4” at 200 yards

John

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CBA Member

joeb33050 posted this 26 March 2016

John Alexander wrote: The fatal flaw in my example in post #49 is that my assumption that the sight line and bore line converge at 200 yards.

I don't think so. The fatal flaw in all the discussions about cant is the failure to recognize that when the sights misalign because of cant, or anything else, that the shooter will re-align the sights. I got hung up on the arithmetic, describing a situation that will not exist. The shooter will align the sights, cant or not, and the bullet will hit center. Canted or level, with the sights aligned, the bullet hits center.

The arithmetic describes a situation where the rifle is canted, the sights are misaligned, and the shooter shoots anyhow.

The arithmetic goes on to talk about canted rifle, misaligned sights and shooting at other than the zeroed range.

If a rifle is canted, and if the sights are aligned, and if the range shot at = the sight-in range; then the bullet will strike center. (Or as close as it does with a level rifle.)

Scope and irons, same story. Ten minutes with pencil and paper convinces anyone.

This is a wheels-on-luggage moment.

I just pulled that out of the air arbitrarily. Unfortunately that is totally unrealistic for real rifles especially cast bullet rifles. For instance in Joe's load which drops 9.5 inches in 100 yards the sight and bore line would have to cross at 21 yards for a 100 yard zero making all my computed deflections much much bigger. For zeros for longer ranges the point where the sight line and bore line cross would be even closer to the muzzle making the deflections bigger and more important yet.

As usual, it it those darned assumptions that screw up all the nice logic or math that follows.

John

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CBA Member

joeb33050 posted this 26 March 2016

I don't think so. If you move your gun to align the sights, it's at the same point of aim. Your sentence below is correct. If you always roll your rifle to get the crosshairs in place, a change in shoulder pressure may have moved your rear bag a few thousands of an inch and you will miss it, fire shot number 2 and then repeat the above.

We don't need a reference like the top of the target to level the rifle. I've shown this with test results that can be easily duplicated.

If you raise your rifle to the edge of the target to regain level, you have potentially moved both rests and the rifle. See the paragraph above.

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CBA President

John Alexander posted this 26 March 2016

The fatal flaw in my example in post #49 is that my assumption that the sight line and bore line converge at 200 yards. I just pulled that out of the air arbitrarily. Unfortunately that is totally unrealistic for real rifles especially cast bullet rifles. For instance in Joe's load which drops 9.5 inches in 100 yards the sight and bore line would have to cross at 21 yards for a 100 yard zero making all my computed deflections much much bigger. For zeros for longer ranges the point where the sight line and bore line cross would be even closer to the muzzle making the deflections bigger and more important yet.

As usual, it it those darned assumptions that screw up all the nice logic or math that follows.

John

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Scearcy posted this 25 March 2016

Ken YOU ARE CORRECT SIR! When I get home from my current winter break in Joe's fine state, I'll give it a try. Hopefully someone will beat me too it. Jim

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Mike H posted this 25 March 2016

Speaking about long range hand held rifles.1,000 yard range,nominal 30minutes of elevation from zero30 minutes @ 1,000 yards = 300"Radius of 300” gives a diameter of 600” or 50 feetCircumference becomes 1,884"360 degrees in a circle,1884” divided by 360 = 5.2" If you look at a clock face,a minute doesn't look to be very far,divide that by 6 to get one degree,a lot less,now take stress,fatigue and all the concentration used to fire a shot,it is not hard to get some cant.I will keep using my level.Mike.

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joeb33050 posted this 25 March 2016

JHS wrote: Here is the scenario as it would play out for me. I fire my first of five shots for a group. I am shooting in a match. I push my rifle forward into battery again. The cross hairs are 1/2” right of the dot. Everything feels right but obviously something moved a little. Do I: Move the rear bag slightly? Turn the windage screw on my front rest? Check the level on the scope before either of the above? Fire a sighter (different bull) and then repeat the above? I will check the bubble in the level. The majority of the time, the cross hairs will come right back into alignment. If you touch the front screw and your rifle is canted slightly you will have unconsciously moved your point of aim. If you always roll your rifle to get the crosshairs in place, a change in shoulder pressure may have moved your rear bag a few thousands of an inch and you will miss it, fire shot number 2 and then repeat the above. If you raise your rifle to the edge of the target to regain level, you have potentially moved both rests and the rifle. See the paragraph above.Agree. However, some are talking about LR rifle, which, as you know, ain't about what you're talking about. However, watching the level and removing cant works too, as you said. What doesn't work and doesn't happen, is canting the rifle, seeing the cross hairs off the center, and shooting anyhow. Nyet?

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Ken Campbell Iowa posted this 25 March 2016

hi guys ... please don't grab your 22 squirrel rifle and run out in your back yard and shoot 2 of 3 shot groups ....one scope straight up, one rifle rotated 90 degrees on the bench . show us the target .

please don't do this .. it would break the mood ...

heh

ken

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CBA Member

Scearcy posted this 25 March 2016

Here is the scenario as it would play out for me. I fire my first of five shots for a group. I am shooting in a match. I push my rifle forward into battery again. The cross hairs are 1/2” right of the dot. Everything feels right but obviously something moved a little. Do I: Move the rear bag slightly? Turn the windage screw on my front rest? Check the level on the scope before either of the above? Fire a sighter (different bull) and then repeat the above? I will check the bubble in the level. The majority of the time, the cross hairs will come right back into alignment. If you touch the front screw and your rifle is canted slightly you will have unconsciously moved your point of aim. If you always roll your rifle to get the crosshairs in place, a change in shoulder pressure may have moved your rear bag a few thousands of an inch and you will miss it, fire shot number 2 and then repeat the above. If you raise your rifle to the edge of the target to regain level, you have potentially moved both rests and the rifle. See the paragraph above.

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CBA Member

joeb33050 posted this 25 March 2016

Let's see if this is correct. (I'm using up a lot of paper drawing.) Telescopic sights, John's example, 2” and 2 degrees The rifle is sighted in at a certain range. The rifle is canted-to the right. The cross hairs move right .07” and down .001". Now, the shooter who cants the rifle doesn't shoot with the scope aimed off, he moves the rifle/scope.

The shooter moves the butt of the rifle/scope to the right, moves the rifle/scope counter-clockwise viewed from the top, aligning the cross hairs and target once again.

The shooter also moves the butt of the rifle/scope up to center the cross hairs.

The canted rifle/scope now has the scope aligned with the target.

The displacement of the perfect bullets/perfect gun is now:

(Actual Range/Sight In Range) X Cant movement, W and E

So if the right movement is .07", and the sight in range is 200 yards, at 400 yards the displacement is (400/200) X .07” = .14", and at 100 yards is (100/200) X .-7” = .035".

At the sight in range there is NO displacement.

Note that it does NOT matter where the scope is located with respect to the rifle. The scope can be hooked to an apparatus and be three feet left and two feet high of the bore center, and with two targets placed properly, shot accurately. Aim at one, hit another. My 223...

Note that having the scope horizontal hair LEVEL when the rifle is held as the shooter wants, insures ONLY that W and E changes operate only on W and E. Think about a scope mounted so cross hairs form an X.

If the rifle is sighted in at the range the shooter wants to shoot at; and If the shooter moves the sight alignment to compensate for any cant-scope-movement; then cant means no displacement.

Then why the levels?

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R. Dupraz posted this 25 March 2016

''Given the amount of sight adjustment (elevation) needed on a BPCR to go from 200 to 1000 yards and the height of the midrange it's no wonder a spirit level is necessary for accurate shooting" Been there.

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Larry Gibson posted this 25 March 2016

What also needs to be considered is that bullets do not travel in straight lines. Given a match .308W load at 2650 - 2700 fps it requires 35 +/- moa to go from a 200 yard zero to a 1000 yard zero. That changes the mid range from +1 3/4 - 2” to 12' +/-. A 1 degree cant moves the impact 5” at 1000 yards . The 2 degree cant in John's calculation would actually put the bullet 11+” to the side and low.

Given the amount of sight adjustment (elevation) needed on a BPCR to go from 200 to 1000 yards and the height of the midrange it's no wonder a spirit level is necessary for accurate shooting.

LMG

Concealment is not cover.........

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Scearcy posted this 25 March 2016

Joe and John Help me out here. In Joe's example the bullet drops 9.5 inches in 100 yards. Hence our scope is adjusted to offset that drop in order to place the point of impact on the point of aim. While it would never happen, I think it is useful to consider what would happen if the rifle were canted 90 degrees. If we ignore the effect of gravity, the bullet will strike 9.5 inches to the right of the point of aim. That is how the rifle was adjusted when it was oriented vertically. Gravity will only serve to drop the bullet 9.5 inches down as it always will - regardless of cant, in this example. Now for a very small cant, the vast majority of the effect of gravity is offset by the initial scope setting as it always is. I agree that lets ignore this for simplicity. The question is, what is the horizontal deflection. Assuming your formulae are correct, my disagreement is with the radius to which you are applying your geometry. It seems to me the radius is 9.5 inches rather than 6 inches or 2 inches. This would make Joe's deflection numbers even larger. The real deflection with which we are concerned here is the axis of the bore. When the shooter reorients the point of aim back to the bull without correcting the cant, the axis of the bore remains on an arc of 9.5 inches but oriented 1 degree clockwise from vertical. What am I missing?

Jim

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CBA President

John Alexander posted this 25 March 2016

Joe,

I think it is but I have been wrong once or twice before. My reason is in this assumption which I believe is wrong:

"As the rifle is rotated the impact point of the bullet moves in a circle of radius = drop of the bullet and center at the zero cant impact point - drop. Let's say that the drop is 6” over some range, and you have zeroed the rifle with no cant. Canting the rifle moves the bullet through a circle of 6” radius, and the center of the circle is 6” below the beginning, zero cant impact point." The radius involved when the rifle is canted does not include the drop of the bullet. If it did canting the rifle would swing the trajectory away from straight down (gravity). Canting the rifle can't affect trajectory except by where the muzzle is and what direction it is pointing then the bullet emerges. In a canted rifle the muzzle isn't directly under the scope axis (0.07” in the example). The direction is a bit different because the convergence angle between sight line and bore has rotated a bit from vertical.

We could argue a bit about the radius. If you assume the rotation is about either the line of sight (keeping your eye on the target shooting in prone) or about the bore, the radius is 2” inches in my example. If you assume that the center of rotation is where the stock rests on the bags then the radius would be the distance from the bags to the bore line -- maybe still near 2” or maybe a bit more depending on the shape of the stock.

John

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joeb33050 posted this 25 March 2016

John Alexander wrote: This discussion has gotten a lot more interesting than I expected. When it went from “is a spirit level better for controlling cant than the horizontal crosshair” to the the question of how important, or not, a few degrees of cant is and is that importance, or lack of importance, only at long range. I will admit that my head was spinning at times until I thought -- wait this is a geometry problem and I had that as a sophomore in high school. This isn't like trying to figure out what really happens to a cast bullet in the throat when the gun goes off. We don't have to rely on guesses, opinions, what is written somewhere by an authority (who may be wrong) or “what everybody is doing.” I think I finally have the geometry problem straightened out in my head and you can explain to me why not if I am wrong. Assume we have a long range rifle with the long axis of the scope 2 inches above the axis of the bore and the axis of the scope (sight line) and the axis of the bore converging at 200 yards. (for what distance that convergence would zero the rifle would depend on velocity and BC of the bullet but is irrelevant for now ”€œ forget what the bullet is doing until later.) Now assume that the rifle is canted 2 degrees from where it was uncanted the shot before. Joe's experiments showed that shooters are amazingly good at eliminating cant, so 2 degrees is a lot. By geometry (2” x sin 2 degrees) this puts axis of the bore 0.07” horizontally from where it was for the last shot in relation to the scope axis (no matter the center of rotation.) The vertical deflection (2 ”€œ 2 x cos 2 degrees) = 0.001” so we can ignore the vertical change. Both of these deflections are at the rifle and bench.

I GET THE SAME NUMBERS, .0698 AND .00122. This will mean that although the sight line and axis of the bore will still converge at 200 yards, at 400 yards the bore axis will be located .07” to the opposite side that it was deflected towards back at the rifle. At 1,000 yards the axis of the bore will be 0.28 inches from where it was for the uncanted shot before. YES, BECAUSE THE X HAIR CENTER IS ROTATING ABOUT THE ORIGINAL CENTER OF ROTATION, THE CENTER 2” FROM THE X HAIRS.

Where is the bullet? In some previously posts it was mentioned, or assumed, that the “trajectory was canted” when the rifle was canted. This can't be.

AGREE

Except for minor influences (latitude, twist, and direction rifle was pointed) and the usual major influence of wind, the trajectory will go relentlessly toward the center of mass of the earth (gravity) no matter how you cant the rifle. If the analysis above is right then the error caused by inconsistent cant is equally important at all ranges ”€œ not much at all at any.

For instance the error at 1,000 yards caused by a 2 degree change in cant is about a quarter of an inch (0.7” for a yough 5 degree cant). Needless to say at a distance where the wind deflection may be varying in feet from one shot to the next this isn't the shooters biggest problem. If there are errors in my thinking or calculations please let me know. ONE OF US IS WRONG HERE. I WROTE: More precisely, cant is the rotation of the rifle about the axis going through the sights and the center of the target. The shooter will keep the sights aligned with the center of the target, and canting will rotate the rifle about the line going through the sights and to the target. As the rifle is rotated the impact point of the bullet moves in a circle of radius = drop of the bullet and center at the zero cant impact point - drop. Let's say that the drop is 6” over some range, and you have zeroed the rifle with no cant. Canting the rifle moves the bullet through a circle of 6” radius, and the center of the circle is 6” below the beginning, zero cant impact point. Let R = radius and C = the cant angle in degrees, then horizontal deflection of the impact point = R Sin C and vertical deflection = -R-(R Cos C.) The movement of the impact point is a function of the bullet drop and the angle of cant. For any cant angle up to 10 degrees or so, the amount of vertical shift is a small fraction of the horizontal shift, and can be ignored for most applications. Drop is a function of muzzle velocity and the Ballistic Coefficient (BC) of the bullet, and is easily estimated on a personal computer using one of the ballistics programs that are readily available. A plain base cast bullet has a typical BC of .4, and is fired at around 1400 feet per second (f/s.) That bullet will drop 9.5” in 100 yards and 1449” in 1000 yards. Canting the rifle to the right, for example, moves the impact point to the right and down thus: Cant (degrees) 100 yd 100 yd 1000 yd 1000 yd right” down” right” down” 1 .166 .001 25.3 .221 2 .332 .006 50.6 .883 3 .497 .013 75.8 1.986 4 .663 .023 101.1 3.530 IS IT ME? John

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CBA President

John Alexander posted this 25 March 2016

             This discussion has gotten a lot more interesting than I expected.&nbsp; When it went from “is a spirit level better for controlling cant than the horizontal crosshair” to the the question of how important, or not, a few degrees of cant is and is that importance, or lack of importance, only at long range. &nbsp;I will admit that my head was spinning at times until I thought -- wait this is a geometry problem and I had that as a sophomore in high school.&nbsp; This isn&#39;t like trying to figure out what really happens to a cast bullet in the throat when the gun goes off.&nbsp; We don&#39;t have to rely on guesses, opinions, what is written somewhere by an authority (who may be wrong) or “what everybody is doing.” I think I finally have the geometry problem straightened out in my head and you can explain to me why not if I am wrong.

Assume we have a long range rifle with the long axis of the scope 2 inches above the axis of the bore and the axis of the scope (sight line) and the axis of the bore converging at 200 yards. (for what distance that convergence would zero the rifle would depend on velocity and BC of the bullet but is irrelevant for now ”€œ forget what the bullet is doing until later.) Now assume that the rifle is canted 2 degrees from where it was uncanted the shot before. Joe's experiments showed that shooters are amazingly good at eliminating cant, so 2 degrees is a lot. By geometry (2” x sin 2 degrees) this puts axis of the bore 0.07” horizontally from where it was for the last shot in relation to the scope axis. The vertical deflection (2 ”€œ 2 x cos 2 degrees) = 0.001” so we can ignore the vertical change. Both of these deflections are at the rifle and bench. This will mean that although the sight line and axis of the bore will still converge at 200 yards, at 400 yards the bore axis will be located .07” to the opposite side that it was deflected towards back at the rifle. At 1,000 yards the axis of the bore will be 0.28 inches from where it was for the uncanted shot before. Where is the bullet? In some previously posts it was mentioned, or assumed, that the “trajectory was canted” when the rifle was canted. This can't be. Except for minor influences (latitude, twist, and direction rifle was pointed) and the usual major influence of wind, the trajectory will go relentlessly toward the center of mass of the earth (gravity) no matter how you cant the rifle. If the analysis above is right then the error caused by inconsistent cant is equally important at all ranges ”€œ not much at all at. For instance the error at 1,000 yards caused by a 2 degree change in cant is about a quarter of an inch (0.7” for a yough 5 degree cant). Needless to say at a distance where the wind deflection may be varying in feet from one shot to the next this isn't the shooters biggest problem. If there are errors in my thinking or calculations please let me know. John

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billwnr posted this 25 March 2016

When I shot NM Highpower my front sight had a level on the back of it. Prevented “off shots” from canting the rifle.

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gpidaho posted this 25 March 2016

Oh Brodie,if you only knew! The voices in my head have more fun in there than I have out here! Gp

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Brodie posted this 24 March 2016

gpidaho, Your mind just isn't screwed up enough to make sense of the concept. Brodie:wavw2:javascript:emoticon(':wavw2:',%20'images/emoticons/party0018.gif')

B.E.Brickey

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gpidaho posted this 24 March 2016

After a couple hours more thinking to much the dim bulb came on. I can now understand that the relation of the D&T only has to correspond to the given. That being the V and thanks for your thoughts and patience. I'll move on to obsessing over something else now! lol Gp

Juggle levels on rifles

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