Advanced Marksmanship 90 degree left or right cant

BOL556

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Dec 7, 2009
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Hey guys

Went to my first long range shooting match over the weekend. One of the courses of fire was shooting with the rifle either on the right or left side at 90 degrees. Target was 6"x6" at 150 yards. How would you set your scope up to hit the target with the rifle laying on his side like that. I recently read an article saying when you put the rifle in that position your windage turns into elevation and your elevation turns into your windage. Wondering if any one could break it down barney style so a beginner would understand how to make the shot.

Thanks for your replies in advance.
 
Re: 90 degree left or right cant

The way I understand it, that is essentially correct. If the rifle is 90 degrees, elevation knob will move the bullet left-right and windage adjustments will move bullet up-down. I am no expert, though. As far as making the shot adjustments, alot will depend on your zero. If its hitting 6x6 steel at 150 and your zero is at 100, little adjustment ,if any would be needed.
 
Re: 90 degree left or right cant

You have to account for the change in mechanical offset, or the height of the scope over the bore, which changes it from a straight up and down difference.

It's not as simple as using the elevation knob and windage knob, but it is not hard either, most just hold the difference using the reticle.
 
Re: 90 degree left or right cant

I wouldn't suggest setting up the scope for it, because that will screw up your zero.

Assume that we’re talking about a 100 yard shot:

The elevation zero is comprised of two parts: First is compensation for the bullet drop from the muzzle to 100 yards. With a .308 that drop is about 2.7 inches, or 2.6 MOA or .75 Mils. Second is the mechanical offset of the line of sight over the line of the bore. Assume my sight height of 1.75 inches, which at 100 yards is about 1.7 MOA or .5 Mils.

Add those two up, and you get about 4.3 MOA or about 1.25 Mils.

If you rotate your rifle to the left, which is bolt-up for a right-handed rifle, your windage is now your elevation (but it’s at zero). And your elevation zero is now your windage zero, but it’s about 1.25 Mils to the right of the line of bore.

So, if you fire a shot at point of aim, with no adjustments on the scope, the point of impact will be about 1.25 Mils left of the point of aim because that is how far the scope is off from the line of the bore. And the shot will be about .75 Mils low because there is no longer a sight height to compensate for since - with the rifle sideways - the scope is at the same level as the bore.

Therefore, for this kind of shot, you hold to the right by about 1.2 Mils and high by about .75 Mils, you should be pretty close to the target.

Of course, if you are making a bolt-up shot at a distance of greater than 100 yards it gets more complicated. In that case you might want to establish a parallel bore zero. Then you would have to hold to the right by the amount of the sight-height, and hold high by whatever the actual bullet drop is at that distance.

[Thanks to Lindy for explaining all this to me a long time ago, before I really understood it.
smile.gif
]
 
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Re: 90 degree left or right cant

Graham explained the fundamentals nicely.

Now, if you think you'll see this again, go to JBM and print out a data sheet with the cant value set to -90 degrees. I wouldn't bother printing data out past 500, as I think it would be rare to be asked to make such a shot. But I have done it to 500 in a match.

As long as you understand what Graham posted, you can take the JBM data and make the correct holds and make the shot. It's never a bad idea to confirm data from JBM, and when I do that I dial to be sure I'm not messing up the hold. But in a match where I don't want to get my scope a rev off zero, I hold for this shot.
 
Re: 90 degree left or right cant

So example time. My scope is 1.5" over bore, scope is in MOA with a mil dot reticle, Zero is 100 yards. If I aim right 1.25 using the mil dot reticle and up roughly about half a mil I should be able to hit it. I am more of a visual learner. I am also going to try this on wednesday when I go the range. Hopefully what I said above would do the trick. What is the formula to go from inches to MOA. This would also help out to be able to convert the bullet drop and adjust for scope height.
 
Re: 90 degree left or right cant

Interesting, might have to get some practice with this out at the range. Good explanation... Thank you.
 
Re: 90 degree left or right cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">I wouldn't suggest setting up the scope for it, because that will screw up your zero.

Assume that we’re talking about a 100 yard shot:

The elevation zero is comprised of two parts: First is compensation for the bullet drop from the muzzle to 100 yards. With a .308 that drop is about 2.7 inches, or 2.6 MOA or .75 Mils. Second is the mechanical offset of the line of sight over the line of the bore. Assume my sight height of 1.75 inches, which at 100 yards is about 1.7 MOA or .5 Mils.

Add those two up, and you get about 4.3 MOA or about 1.25 Mils.

If you rotate your rifle to the left, which is bolt-up for a right-handed rifle, your windage is now your elevation (but it’s at zero). And your elevation zero is now your windage zero, but it’s about 1.25 Mils to the right of the line of bore.

So, if you fire a shot at point of aim, with no adjustments on the scope, the point of impact will be about 1.25 Mils left of the point of aim because that is how far the scope is off from the line of the bore. And the shot will be about .75 Mils low because there is no longer a sight height to compensate for since - with the rifle sideways - the scope is at the same level as the bore.

Therefore, for this kind of shot, you hold to the right by about 1.2 Mils and high by about .75 Mils, you should be pretty close to the target.

Of course, if you are making a bolt-up shot at a distance of greater than 100 yards it gets more complicated. In that case you might want to establish a parallel bore zero. Then you would have to hold to the right by the amount of the sight-height, and hold high by whatever the actual bullet drop is at that distance.

[Thanks to Lindy for explaining all this to me a long time ago, before I really understood it.
smile.gif
] </div></div>

Wow, that really got me thinking about this. It makes sense but I have a few comments / questions?

1) If you were only shooting at 100 yards and wanted the left/right point of impact (POI) to match the point of aim (POA), then I would think you only need to lower the elevation by the gravity component (2.6 MOA or .75 mils per the example). This would leave the mechanical offset of 1.75" to match the POI to the POA (at least horizontally).
2) To get a parallel scope line of sight to the bore, in the right hand bolt up configuration, wouldn't you need to remove both elevation components discussed above? So lower the elevation knob (before you roll the rifle) by 4.3 MOA or 1.25 mils per your example? I would think so...
3) With the parallel bore case, and the right hand bolt up configuration, the POI would be 1.75" RIGHT of the POA, since the scope is 1.75" left of the bore. So you would have to hold 1.75" LEFT, not right. Note, that this 1.75" is an actual distance unit, not angular unit like IPHY, MOA, etc.

So if I've thought about this correctly, here are the steps I'd take to get a parallel bore, and get the proper holds.

(for the right hand bolt up configuration)
1) Dial elevation DOWN for the scope height and the gravity drop for your zero. 4.3 MOA or 1.25 mils per your example. This gets the parallel bore to scope.
2) Dial wind RIGHT for the gravity drop for your zero. 2.6 MOA or .75 mils per the example. This gets your new elevation set for your zero distance.
3) Roll rifle left, bolt up for right handed bolts.
4) Hold 1.75" (or whatever your scope offset is) LEFT via the reticle.
5) Hold UP for shots past your zero based on your standard dope , or dial via the original wind knob (dial RIGHT).

I guess you'd have to remember where your 'zero' is on your wind knob (it probably won't be on the printed 0), if you are going to dial for elevation. Remember how many rotations you've made, etc...

Seems like a lot of work. How often are these shots mandated at a competition? I've never heard of one.
 
Re: 90 degree left or right cant

I had the same shot at the SniperGolf comp two weekends ago and it was 190 yds if I recall correctly. My plan was to hold 1.5 mils right and 1.5 mils high. I was .5in low and .5in left on my first shot. So I corrected by holding another .5mil high and .5 mil right. I ended up with a perfect two shot group with both bullet holes touching and missing my point of aim. I guess I did something screwy with the trigger on the second shot. I'll need to practice this one to see what kind of errors I'm making over say 20 shots.
 
Re: 90 degree left or right cant

I was at the Sniper Golf match as well. If I remember correctly the range I was told was 150 yards. I am also guessing the the shamrocks were roughly 4"x4". I was "that guy" with the canted scope on the FN SPR A3G. The good thing is that the scope issue is fixed and cant wait to go back to redeem myself.
 
Re: 90 degree left or right cant

I have a T-1 micro offseted at the 3 o'clock position. When the rifle is laying on its right side, the t-1 is upright. Now my question is I could just zero the t-1 at a 50 yd zero and should be good to 250 in this position correct? I use m118lr for ammo.
Would I just input 90 degree can't into JBM and T-1 sight height to calculate a general rule of thumb range card for a position like this.
 
Re: 90 degree left or right cant

To me with the T1 being mounted at the 3 o clock you wouldnt have to compensate for anything. You would just have to input the height over bore if you really wanted to. Being a red dot you may also be able to zero at 25 yards which will allow you to go out to 300 yards. Not sure but if the 308 flies the same or about the same as a 556 with in those 300 yards you would be good.
 
Re: 90 degree left or right cant

If you are zeroed at 25 yards with the rifle canted 90 degrees, you will have a lot of windage error at distance.

If that is the way you want to go, zero the red dot at 25 yard with the same offset as height over bore distance. The impact should be right of the POA by the same distance as the height over bore.
 
Re: 90 degree left or right cant

Shooting a GAP HRT with a USO w PCMOA ret. I can hold or dial approx 2.5 MOA down and 2.5 MOA right or left depending on cant direction (@ 100 yards)!

My sight height is 2.0 inches over bore!

Zero is 100 yards.

2.5 MOA down for elevation correct right or left cant!

2.5 MOA right for bolt up cant (right hand gun)

2.5 MOA left for bolt down.
 
Re: 90 degree left or right cant

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Here is a picture of me checking my port side zero.

-Dial 3 MOA right windage
-Hold 3 MOA left with the rifle on "port" (left) side.
 
Re: 90 degree left or right cant

I plan on trying this out tomorrow from 100-300

Using BulletFlight with my existing data for my rifle..

1. Set "zero" range as short as possible, which on this program is 25 yards.

2. View drop to your actual "zero". This value will need to be dialed down to establish a close to "Parallel to bore" setting.
(This setting is valid for a 90* cant in either direction)

3. Refer to your same "25 (X) yard zero" dope table to get inclination to apply to your windage knob (now your elevation).

4. Hold over for windage what ever your mechanical offset is (height over bore),(I'm thinking a hold-over will be more practical due to changes in distance will alter this value if it is compensated for by dialing into the optic, not to mention a H.O.B. of only a few inches becomes insignificant at 300+ yards).

Example - Height over bore of 3" equates to .830 mils at 100y
.416 mils at 200y
.270 mils at 300y
.208 mils at 400y
 
Re: 90 degree left or right cant

I've calculated mine, then shot it and corrected to find the true 100yd dope. What I found was I needed more correction than the calc. My supposition is that the formula doesn't account for barrel droop. There is a guy by the name of Varmint Al with some analysis posted on the web on barrel droop, which seems to account for my extra adustment needed.

For my 308, the sight height is 2.1 inches. A JBM calc shows that it should drop 2.6in at 100yd. Set zero range to 1 yd, and sight height to 0 and find the drop at 100yd. So the calc says the windage should be set to correct for 2.6" only, which is approx 2.5MOA. But my actual windage correction is 3.0MOA. I think the extra .5MOA is barrel droop, where it points lower so you need more windage (now elevation) correction. My actual elevation correction is 3.5MOA. JBM shows 2.6" drop again, with my 2.1" sight height, should be 4.7" at 100yd which is approx 4.5MOA. My actual dope is 3.5MOA. Which again, if barrel droop is a factor, the barrel quits drooping when flipped sideways, so points "higher" in the elevation direction, therefore needing less correction than what was calculated.
 
Re: 90 degree left or right cant

Start with some imaginary lines for a visual to help you understand sights and zeroing: line of bore/departure, line of sight, and bullet path. No matter where the sights are located on the rifle, as long as they are in the same plane as the bore, and are the same height above line of bore, allowing, from such condition, wind-age and elevation adjustment, sighting in and zeroing will be possible. And, when the rifle is sighted in, whereby the barrel angle is greater than horizontal, the bullet path will intersect the line of sight twice. This allows rifles like the M4 to be sighted in at 25 meters for a zero at 300 meters. Remember too, for an offset sight, when this sight is the one being used it is not the offset sight, it's the one not being used which is canted. If you do shoot with the canted sight in a canted condition zeroing at anything other than distances where the target could be hit intutitively will require motor memory development to get a consistent cant.
 
Re: 90 degree left or right cant

aagifford - You didn't have to go through all of that math to get data from JBM. Leave the zero set to 100 yards, and set the cant to 90 degrees.

Try that, then see if those numbers are closer to your actual data or closer to your calculated values.
 
Re: 90 degree left or right cant

Good suggestion, I've exercised JBM a bit trying to get results that make sense, and then drew a diagram to understand the mechanics of it better. I found that the windage and elevation correction should both be the same number. That number is .6mil (my hand calc shows .58 mil). So, .58mil elevation, and .58mil windage.

My actual for my 243, which has a 1.9" sight height (I just went and found my data book for it) is 1 mil down and 1.1mil right at 100yd. I have a long, heavy barrel, and I think the barrel bends a bit, and the barrel flexes the action, which combine together to increase the needed corrections.
 
Re: 90 degree left or right cant

Thinking about it a little more, the sight height drops out of the equation whenever the zero range is the same as the target range when canted 90°. As the range when canted is moved either way from the zero range, the sight height will come into play, and start drawing the shot away from the zero range in elevation direction.
 
Re: 90 degree left or right cant

Something else to think about. One reason we shoot with sights level is that the eye naturally balances and centers things. When alignment is perceived as being identical (balanced and centered) consistently, misalignment is reduced. That's good since misalignment introduces angular error, which increases with distance. Another reason to shoot sight level is that consistent recoil resistance is better assured which also improves accuracy. Multiple sights on the rifle with one set canted sounds like a godsend and somewhat novel too. But, when the shooter has mastered a BDC iron sight set there's little point in the additional sight. The exception would be for an extremely talented shooter with a full time job in the shooting sports.
 
Re: 90 degree left or right cant

Data for my .308


SBU 0K 10 MPH
YARDS DROP WIND
50.0 -0.3 0.1
100.0 -0.7 0.7
150.0 -1.1 0.9
200.0 -1.5 1.0
250.0 -1.9 1.1
300.0 -2.4 1.1
350.0 -2.8 1.1
400.0 -3.3 1.1
450.0 -3.9 1.2
500.0 -4.4 1.2
550.0 -5.0 1.2
600.0 -5.7 1.2
650.0 -6.4 1.2
700.0 -7.1 1.2
750.0 -8.0 1.2
While not all of this data is confirmed, a few distances have been. Enough that I trust it to hit IDPA size targets.

Shoot it beyond 100 yards and you'll get a better feel for it.
 
Re: 90 degree left or right cant

Math will get you close, but you will have to shoot it to know for sure.

My bolt up dope is different from my bolt down dope.

I have 2 comp rifles set up identically, one in .223 and one in .243 and both rifles are exactly the same dope, ie, different with bolt up and down.

This shows me I am seeing/holding/gun recoiling different on the 2 different sides, ie. bolt up or down.
 
Re: 90 degree left or right cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hellbender</div><div class="ubbcode-body">My bolt up dope is different from my bolt down dope.</div></div>How it is different is what, in police work, we call a 'clue':
What happens when your windage zero becomes your elevation reference?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hellbender</div><div class="ubbcode-body">This shows me I am seeing/holding/gun recoiling different on the 2 different sides, ie. bolt up or down. </div></div>Does it really? Is that conclusion supported by the evidence or could the difference in POI be due to something else?

I see people posting about line of sight and line of bore and bullets crossing the intersection twice - basically all that ground-level 1980s sketch-illustration stuff about trajectory - but that's not what we are talking about here. Go back and read my above post. The issue is what happens to a zero when the plane is turned ninety degrees. An understanding of reverse offset is necessary in terms of how a zero is derived, but we are not yet working at distances shorter or longer than the zero range.
 
Re: 90 degree left or right cant

Graham is spot on with his original post. Very detailed. A little simpler I think:

First of all, it's important to understand that you have en elevation adjustment accounted for in your zero. Just because your elevation turret is set to 0 doesn't mean that there has been no adjustment made for elevation. Now to the windage knob. The windage knob is at an actual zero adjustment setting when it is set to zero (assuming you are shooting a properly zeroed rifle).

You rotate your rifle 90 degrees,

You just turned your elevation adjustment into a bad windage adjustment, i.e. you hit left (for a bolt up 90). You have also lost the elevation adjustment that you had built into your zero, i.e. you hit low. Your new shot group will be around a mil low and a mil left. There will be minor deviations to that due to differences in bore height and caliber from rifle to rifle. The best way to know for sure is to go to the range and shoot this position. Once you have shot a 90 degree group, make the necessary elevation adjustment (this will be on your windage knob since it is now controlling your elevation adjustments) to put you online with your horizontal crosshair. You can now do mil-holds more easily for further shots.

Note: On many scopes the elevation knob will not go far enough below zero to pull you back to the vertical crosshair in your scope. For this reason, making a windage adjustment on the 90 degree group was omitted.
 
Re: 90 degree left or right cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">I wouldn't suggest setting up the scope for it, because that will screw up your zero.

Assume that we’re talking about a 100 yard shot:

The elevation zero is comprised of two parts: First is compensation for the bullet drop from the muzzle to 100 yards. With a .308 that drop is about 2.7 inches, or 2.6 MOA or .75 Mils. Second is the mechanical offset of the line of sight over the line of the bore. Assume my sight height of 1.75 inches, which at 100 yards is about 1.7 MOA or .5 Mils.

Add those two up, and you get about 4.3 MOA or about 1.25 Mils.

If you rotate your rifle to the left, which is bolt-up for a right-handed rifle, your windage is now your elevation (but it’s at zero). And your elevation zero is now your windage zero, but it’s about 1.25 Mils to the right of the line of bore.

So, if you fire a shot at point of aim, with no adjustments on the scope, the point of impact will be about 1.25 Mils left of the point of aim because that is how far the scope is off from the line of the bore. And the shot will be about .75 Mils low because there is no longer a sight height to compensate for since - with the rifle sideways - the scope is at the same level as the bore.

Therefore, for this kind of shot, you hold to the right by about 1.2 Mils and high by about .75 Mils, you should be pretty close to the target.

Of course, if you are making a bolt-up shot at a distance of greater than 100 yards it gets more complicated. In that case you might want to establish a parallel bore zero. Then you would have to hold to the right by the amount of the sight-height, and hold high by whatever the actual bullet drop is at that distance.

[Thanks to Lindy for explaining all this to me a long time ago, before I really understood it.
smile.gif
] </div></div>

Wow, that really got me thinking about this. It makes sense but I have a few comments / questions?

1) If you were only shooting at 100 yards and wanted the left/right point of impact (POI) to match the point of aim (POA), then I would think you only need to lower the elevation by the gravity component (2.6 MOA or .75 mils per the example). This would leave the mechanical offset of 1.75" to match the POI to the POA (at least horizontally).
2) To get a parallel scope line of sight to the bore, in the right hand bolt up configuration, wouldn't you need to remove both elevation components discussed above? So lower the elevation knob (before you roll the rifle) by 4.3 MOA or 1.25 mils per your example? I would think so...
3) With the parallel bore case, and the right hand bolt up configuration, the POI would be 1.75" RIGHT of the POA, since the scope is 1.75" left of the bore. So you would have to hold 1.75" LEFT, not right. Note, that this 1.75" is an actual distance unit, not angular unit like IPHY, MOA, etc.

So if I've thought about this correctly, here are the steps I'd take to get a parallel bore, and get the proper holds.

(for the right hand bolt up configuration)
1) Dial elevation DOWN for the scope height and the gravity drop for your zero. 4.3 MOA or 1.25 mils per your example. This gets the parallel bore to scope.
2) Dial wind RIGHT for the gravity drop for your zero. 2.6 MOA or .75 mils per the example. This gets your new elevation set for your zero distance.
3) Roll rifle left, bolt up for right handed bolts.
4) Hold 1.75" (or whatever your scope offset is) LEFT via the reticle.
5) Hold UP for shots past your zero based on your standard dope , or dial via the original wind knob (dial RIGHT).

I guess you'd have to remember where your 'zero' is on your wind knob (it probably won't be on the printed 0), if you are going to dial for elevation. Remember how many rotations you've made, etc...

Seems like a lot of work. How often are these shots mandated at a competition? I've never heard of one.
You are incorrect about having to account for bore height by adjusting left instead of right, you have to adjust right because when you initially zero your rifle with no can’t, you are pushing the sight down to the barrel in order to have an angle on the barrel to account for sight height and, bullet drop (gravity)

so this means when you fire on a right hand bolt at 90 degrees your bullet with go low left if you made no adjustment, as the bullet is leaving a barrel set to cross the recticle after about 15/20 metres and the bullet will drop low as you haven’t accounted for bullet drop.

easiest way to work it out is fire at your zero range, work out your offset to bring sight in line with barrell, then use your wind turret m as elevation and elevation as windage all of which you discussed there was correct.

it is confusing but just remember that sight is zeroed for bullet to pass recticle early in bullet trajectory in order to come back down for POI/POI.
 
You are incorrect about having to account for bore height by adjusting left instead of right, you have to adjust right because when you initially zero your rifle with no can’t, you are pushing the sight down to the barrel in order to have an angle on the barrel to account for sight height and, bullet drop (gravity)

so this means when you fire on a right hand bolt at 90 degrees your bullet with go low left if you made no adjustment, as the bullet is leaving a barrel set to cross the recticle after about 15/20 metres and the bullet will drop low as you haven’t accounted for bullet drop.

easiest way to work it out is fire at your zero range, work out your offset to bring sight in line with barrell, then use your wind turret m as elevation and elevation as windage all of which you discussed there was correct.

it is confusing but just remember that sight is zeroed for bullet to pass recticle early in bullet trajectory in order to come back down for POI/POI.
You joined today to comment on a thread that’s eight years old?
 
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