Advanced Marksmanship How to estimate error due to rifle cant

T

TimK

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Jan 13, 2010
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#1
Been meaning to do the math, here it is.

BD = Bullet Drop
A = Angle rifle is canted from vertical
E = error, left or right

E = BD tan A

Example:
At 425 yards, my .308 drops 31" with a 100 yard zero. If my rifle is canted 5° from vertical, the windage error is:

E = 31" X tan 5°

E = 31" X 0.0875

E = 2.71"

I had a sense the numbers were big, but I didn't know how big. Here's another example because 5° is a lot and 425 yards isn't.

Same rifle at 1,000 yards, bullet drop is 321". Assume cant of only 2°, which seems more realistic.

E = 321 tan 2°

E = 11.2"
 
#2
Re: How to calculate error due to rifle cant

A source I found says horizontal error due to canting may be calculated as:

Error = sin(cant angle) * maximum ordinate of the shot

I find that more believable. If you're interested, one source for that calculation is:

http://www.microlevel.biz/cant_errors.html

(He actually calculates it as cos(90 - cant angle) * max ord, but apparently failed to note that's the same as the above.)

By the way, even with your number at 1000 yards, that's in the same magnitude as the effect of a 1 mph crosswind.
 
#5
Re: How to calculate error due to rifle cant

Until someone explains why "Max Ord" is used instead of total drop, I'll stand by my statement here . I believe that you'll see error on the way "up" and on the way "down"?

I believe cant error explains why some have to hold more/less "wind" when shooting the same loads at the same targets. I'm a believer and user of levels, try to eliminate that error.

Cheers,

Bill
 
#6
Re: How to calculate error due to rifle cant

You guys make my head hurt. It's a wonder I can hit a target at all because all this stuff is greek to me.
smile.gif
 
#8
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: marduk185</div><div class="ubbcode-body">canted rifle= miss ? i like my formula better. </div></div>

LMAO....
 
#9
Re: How to calculate error due to rifle cant

Wouldn't an accurate calculation have to include the distance between the line of sight and bore as one of the constants? The triangle resulting from a tilted rifle has this as one of the sides. Seems that a rifle with the line of sight 1" over the bore would have less horizontal error than a rifle with the L.O.S. 2" over the bore given the same angle.
 
#10
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">If you don't cant the rifle, it doesn't matter. So...don't.
laugh.gif

</div></div>

Perfect - my thoughts exactly! We really don't need to calculate the error if we eliminate it.

There was an idea floating around in my noggin a while back, to rig a "protractor" to the eyepiece and use rifle cant to counteract wind. For example, cant the rifle five degrees "into" the wind for a 10 mph crosswind. Never got beyond the "pondering" stage on that....

Cheers,

Bill
 
#11
Re: How to calculate error due to rifle cant

I don't care about accurately calculating the error, I just wanted to know for myself the relative magnitude. When I ran the numbers, I discovered it's not trivial. I share it only because I had not seen it here before and I thought it was interesting.

Not canting the rifle is the answer, no doubt about it.
 
#12
Re: How to calculate error due to rifle cant

Another distraction...

I equate it to someone telling a group of Pre-SEALs they can pass Buds if they buy the right boots to wear during the training. We can all acknowledge that shoes are an important part physical training, however it is the man not the clothes he is wearing that get through these physically demanding selection courses.

So instead of concentrating on the fundamentals you are looking at your level all the time. Most people don't multi task nearly as well as they think.

Accuracy is not a constant in the real world, a .25 MOA shooter at 100 is not a .25MOA shooter at 1000 -- caveated to say, unless everything lines up exactly right and all things being equal, to which they are not nor are all things ever equal.
 
#13
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">
So instead of concentrating on the fundamentals you are looking at your level all the time. </div></div>

I thought not canting the rifle <span style="text-decoration: underline">was</span> one of the fundamentals.

I don't know how others use their levels, but I look as I'm getting in position, then don't again until the next shot. Doesn't seem like much of a distraction to me.
 
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#14
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tim K</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">
So instead of concentrating on the fundamentals you are looking at your level all the time. </div></div>

I thought not canting the rifle <span style="text-decoration: underline">was</span> one of the fundamentals.

I don't know how others use their levels, but I look as I'm getting in position, then don't again until the next shot. Doesn't seem like much of a distraction to me.</div></div>

Why have a level at all then... if you can tell in the reticle the rifle is not canted, why bother looking the first time... some people have a cant issue, but others also have a trigger issue which is worse than canting as it pull their shots considerably off target... look at the majority of Tactical Practical Targets posted on here, which are shot at 100 and are not duly influenced by cant.

Note the shots off center which, the majority are right by at least .3 MOA at 100 yards... this is a great example because it has good accuracy with a caveat.
angelespictures002.jpg


another of the same:
angelespictures003.jpg



You have to ask yourself, do the majority of your shots miss because of cant or because of something else like demonstrated above... I suggest you review the Practical Target thread and "add up" the misses.

 
#15
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">
Why have a level at all then... if you can tell in the reticle the rifle is not canted, why bother looking the first time...
</div></div>

I see your point now. I <span style="text-decoration: underline">can't</span> tell if the rifle is canted or not just looking through the reticle. I get fooled by the terrain all the time which is why I have a level. I also shoot exclusively off a bipod that tilts and locks. I leave the lock just loose enough to adjust slightly. After I level the rifle, it stays put pretty well. That's what enables me to set it and forget it.
 
#16
Re: How to calculate error due to rifle cant

I would say, calculate the number of misses and the distance from center based on the number of shots taken and then multiple that by distance with a sliding scale of adjustment -- like a drag function to increase the angular dispersion rate so that it grows by at least 1.25X to 1.5X for every 100 yards... then tell me where the cant angle fits into that equation.

Here you can use this target

5090404785_fa7071f923_z.jpg


I personally can tell if the reticle canted even on uneven ground, because the reticle is square to the rifle to begin with -- the question would be have you tried to level the reticle without the bubble then looked to see how far off you were ?
 
#17
Re: How to calculate error due to rifle cant

Actually as a PS... I suppose one would simply need to calculate the average dispersion from center to include center hits based on the number of shots then take that number, multiple it by an increasing factor for distance to decide where these issues like a pulled shot because of improper trigger control would place you, then look at the dispersion from other factors and see where you personal average falls and whether or not you can exploit any of these factors ?

A whole new cottage industry within the shooting community can be grown if you take a 1/4 MOA target that has 50 shots, or 100 shots that require you to shoot them for score. You take the average dispersion and then calculate potential real world input by shooting a similar target at say 300 yards to get your angular dispersion acceleration factor to use which will then give you a multiplier for distance... you can make it part of an App or something. At which point everyone will know whether or not they can exploit sub moa negative effects on accuracy ?
 
#18
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">

I personally can tell if the reticle canted even on uneven ground, because the reticle is square to the rifle to begin with -- the question would be have you tried to level the reticle without the bubble then looked to see how far off you were ? </div></div>

Yeah, this is what turned me on to my inability to do it. I'd level it best I could, then look at the level. I'm always way the hell off.

No doubt there are bigger factors to my misses, this is just one.

I need to read the rest of your posts in more detail and with a fresher brain. The old thinker is pretty worn out at the moment and I'm comprehending poorly.
 
#19
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Wouldn't an accurate calculation have to include the distance between the line of sight and bore as one of the constants? The triangle resulting from a tilted rifle has this as one of the sides. Seems that a rifle with the line of sight 1" over the bore would have less horizontal error than a rifle with the L.O.S. 2" over the bore given the same angle.</div></div>

That's an interesting question, Terry - dammit!
laugh.gif


Let me think about that. I have to fly to D.C. tomorrow, and I'll ponder that question on the airplane.
 
#20
Re: How to calculate error due to rifle cant

We use this rule of thumb:

For every 5 degrees of cant in the rifle you get .1 mil drift every 100 meters in the direction of cant.

ex: 5 degrees cant to the right = .1 mil R at 100 meters
.3 mils R at 300 meters
.6 mils R at 600 meters

2.5 degrees cant right = .05 mil R at 100 meters
.15 mils R at 300 meters
.3 mils R at 600 meters

So, on the range when everyone on the line is hitting a tgt at say 700 meters with a .5 mil hold except that one guy who is holding 1 mil, the first things I check are cant in his rifle and that his windage knob is on zero.
Generally it is one of these 2 issues.
 
#21
Re: How to calculate error due to rifle cant

I could shoot with cant, since I mostly shoot in HP where zeroing for cant is possible. Thing is, the only thing that would suggest using it would be if I could not physically level the gun from the position I'm shootin' from. Sometimes, you're damned if you do and you're damned if you don't. It becomes about muscular tension from a rifle forced to be square vs. muscular relaxation with cant; and, recognizing that the cant may not be consistent, concern about whether the sights are properly adjusted, for whatever the target hold/distance is justified. BTW, the only position for which cant maybe required for most folks is the formal sitting position using a sling support. Sometimes, the prone position with sling support can be a problem too, but, with an adjustable butt it can be solved quickly. Without the convenience of an adjustable stock, if you find yourself relaxed, but canted, muscle applied as consistently as you can make it to square the rifle, may be the lesser of two evils. And, if you're shooting in a "tactical" scenario, by all means, get the rifle square, that's to say, do what L.L. says, he know what he's talking about in that arena.
 
#23
Re: How to calculate error due to rifle cant

I did not buy a level because someone told me it was the cool thing to do or because theory says that cant will do xyz to my POI. I bought it because while firing at an F-class meet at 1000yds i would everyonce in a while get the shot that would pull WAY to the right and low.
I dont keep the level on my rifle because it is cool or someone told me the theory of cant. i keep it there because it fixed my problem.
Yes, there are lots of things that effect a shot so why should you ignore one of those things ? If you instinctually correct for targets sitting on the side of a hill or lying on uneven ground then more powere to ya but I guess I'm not that good.
P.S. When i am setting up to shoot I have both eyes open, glance at the level then continue my routine....it takes less than a millisecond. Shoot a few matches with it and it is second nature. Shooting without that glance would be like shooting but forgetting to pull the trigger.

just my .02$ which probably aint worth much....
 
#24
Re: How to calculate error due to rifle cant

Some ranges/firing points are so off the level that its hard to believe the bubble! Small targets at long range = don't cant. Short practical ranges you probably get away with it.
You can only have a cant if you shoot at a fixed range and can adjust before shots count.
 
#28
Re: How to calculate error due to rifle cant

Unless it is in the scope, or in the visual plane of your non-sighting eye. I can see the level on my Spuhr mount while shooting.

This is a multiple deviation problem, not only are you breaking the verticle allignment of your scope/bore, but you are also canting your verticle and horizontal adjustment lines, so there is a small amount of discrepancy between true and reticle. The flight of the bullet does not change, just the perseption of the shooter, you could turn the rifle 9p degrees and the bullet will fly the same way, but your elevatio and windage will be a little off...its all about what. The shooter thinks is a plummb or level adjustment.

The effect is small, but the best way to deal with it is to just not can't the rifle.
 
#30
Re: How to calculate error due to rifle cant

Depending on one's build, with a standard off the shelf non-adjustable stock, muscular relaxation may not be possible without canting. Also, without some cant, sight alignment may come about only with a very strained neck. I've experimented and prefer a cant for 200 yard sitting in HP. Scores reveal, for me, that's the way to go. However, canting effects my zeros; and, as a practical matter, cant may undermine good hits at UKD targets. My LR rifle has a three way butt (cant, depression/elevation, and pull length). With this sort of butt muscular relaxation is assured with butt canted while the rifle sights are straight up.
 
#31
Re: How to calculate error due to rifle cant

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Terry Cross</div><div class="ubbcode-body">Wouldn't an accurate calculation have to include the distance between the line of sight and bore as one of the constants? The triangle resulting from a tilted rifle has this as one of the sides. Seems that a rifle with the line of sight 1" over the bore would have less horizontal error than a rifle with the L.O.S. 2" over the bore given the same angle. </div></div>

Well, if you want to get technical, how you do the calculation will depend on what kind of cant you're talking about. You can have:

1) The sight canted relative to the axis of the bore (or offset), but level to the horizon.

This is the situation that Sterling Shooter is talking about. Target shooters often do it intentionally, but most of us have this to some degree (but, the effect is so small it's lost in all the other noise). It give you the same effect as an offset optic on a Garand, or to more of an extreme, this: http://ultimak.com/pic36.htm

Cant between the sight and the rifle is undetectable by shooting at your zero distance as long as the sight is level. At any other given distance, the offset will be constant. This will NOT show up in a box test. When you run a box test, it's actually only showing you the cant between your sight and your bubble level, not between your sight and the axis of the bore. A box test will still run square because a vertical adjustment on the sight still moves straight up and a horizontal one still moves straight to the side. You only need to consider the offset of the sight in this situation. Small amounts of cant between the sight and the axis of the bore actually only have a small tiny effect that is lost in the noise, as long as your sight is level.


2) The sight plumb with the rifle, but canted to the horizon.

This is what Lindy's link is showing. It's the situation we are usually talking about when we have these nice discussions about cant. Here, you're not just introducing the error of the sight being offset by tilting it, but your sight adjustments are now also running at an angle. Up is not straight up, it's at the angle of the cant. So, now your scope adjustment from your zero must also be considered. The math is a little easier than the third situation because the cant angle and the sight offset angle are the same. The effect of the offset is dwarfed by the effect of the scope tracking at an angle at longer distances. For small amounts of cant, the effects of the offset due to the sight height are negligible and for simplicity only the effects of the scope tracking at an angle are being considered.


3) The scope canted (or offset) relative the the rifle and also canted to the horizon.

This is the situations where the shooter has intentionally installed the sight to shoot holding the rifle at an angle, but is canting it relative to the angle it is setup at. It is also what you would have when canting the offset Garand optic. Both the offset of the sight to the axis of the bore and the angle the sight is tracking at need to be considered in the calculation. They are now two different angles or an offset and an angle.

2) and 3) could be calculated the came way if you're starting from the sight offsets.

The only one of these I've ever had the need to calculate is 1), the other two are solved with "just don't cant it"... or always cant it consistently.
 
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