We all know that a flatter trajectory helps if there are small range estimation errors. As a precursor to what is coming next, who would like to venture a guess at the truth of the next several statements...???

1) At a given cant (10 degrees for example) a rifle with a trajectory that is 40 percent flatter (boreline drop is the measure) at a given range, will have significantly less Actual Horizontal Error induced at the target at 600 yards.


2) At a given range ( 1000 yards for example ) and inclination of shot (-30 degrees), a rifle with 40 percent less borline drop (40 % flatter) will have less Actual Vertical Error When using the incorrect method of multiplying cosine x yardage as an attempt to compensate for inclination.

3) The faster velocity of a flatter trajectory can make your wind calls better.

1 - T
2 - T
3 - F

WOW!!!!! Brent !!! YOUR ALIVE!!! I thought the terrorists got you????? Just Kidding....

Testing the first two should be quite objective, here is my plan in a nutshell....

For Issue Number one, I will take two sub half MOA rifles with drastically different boreline drop performance at 600 yards. We will zero each rifle at 600 yards with zero cant. Then cant each rifle 10 degrees and fire three shots, then 20 degrees and fire three shots. I will measure the horizontal and vertical deviation of each group relative to its original zero cant group. The first rifle will be an SR25 chambered in 308 win., and the second will be the 30 Wolf. The 308 has 125 inches of borline drop at 600 yards, compared to the 30 Wolf which has 70 inches of boreline drop at 600 yards.

Stay tuned.....

Well, I think I might actually get to the range tomorrow to shoot some. We should have some data on cant at 600 soon.

STL, Nacroswnd, and I got together to do a cant test today. STL shot his 30 Wolf, I shot a .308 Stoner with Fed Gold Medal 168, and Nacroswnd is shooting his 7mm STW with 162 A-max bullets as I write this. All shots were taken at 607 yards per my Vector IV. As you will see the rifles held very well vertically, usually sub 1/4 MOA in less than ideal wind conditions today.

We zeroed the rifles carefully at 607 yards, then fired two shots at a blue piece of tape in the upper left corner of the target, with zero cant on the rifle. We then fired two shots with ten degrees of clockwise cant, and two more shots with 20 degrees of clockwise cant. STL fired first in the most difficult wind conditions of the day. No adjustments were made for wind. When things calmed down, I then followed the same procedure with the .308, and while waiting for the 7 STW ammo to be loaded, in a brief moment of ideal wind conditions, picked a spot much lower on the target and fired two with zero cant, and then a quick shot with 5 degrees of cant. The pictures will be posted, along with the data, tonight or tomorrow.

At the end of the day, we did a blind test to see just how good a shooter is at aligning the rifle in a repeatable fashion. I had STL cant his rifle past 5 degrees, and then look in the scope, and quickly set it as level as he could to take a shot. We then measured the angle and repeated the test over and over. Suffice it to say the STL is very proficient and consistent at leveling his crosshairs just by looking through the scope. On every measurement he was easily between 1 degree and 1.3 degrees. Much more accurate than the average bubble level. I then went through the same thing with my SR-25, the needle never left the black line on ZERO. It is just my opinion, but for skilled shooters with flat shooting rifles, a bubble level is a ridiculous waste of money. You will see what I am refering to when the data gets posted. Of course you should probably get it straight from the horses mouth, but STL hinted that something would have to be seriously wrong with the shooter if he could not see, even 2 degrees of cant when looking through the scope. I concur.

Here is a pic of the data.

You may notice that at 20 degrees and 600 yards, the Wolf had less than 6 inches of vertical change in point of impact, the 308 had 13.5 inches of vertical change from level. The 30 Wolf had 32 inches of Horizontal compared to the 308's 43 inches at 20 degrees cant. The numbers for 10 degrees are also in the table. You will notice that with the Wolf the bullets did not hit lower than level. I think the vertical component of the wind may have influenced this. When the two level shots were taken, there was a slight head wind 2 to 8 oclock. When the ten degree cant shots were taken, there was a slight tail wind about 4:30 to 10:30



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Wolf target, 2 shots at level upper left blue tape, 2 shots at 10 degrees (middle 2 shots) and 2 shots at 20 degrees of cant (far right 2 shots).


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Here is the same target with the SR-25 shooting Fed Gold Medal 168 in .308 caliber. The 308 shots are not circled with the black Sharpie pen. As you can see the error is significantly greater. The vertical error is more than double that of the Wolf at 20 degrees of cant.

At 10 degrees of Cant, the .308 only has 4 inches of vertical change, but induces 26.5 inches of error on the Horizontal axis at 607 yards!

This load has about 20 MOA of borline drop at 607 yards and induces slightly over 4 MOA of Horizontal error with 10 degrees of cant. Could we have a ratio of 5 to one at 10 Degrees??? Where is the guy right out of math class when you need him......Nacroswnd???



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That was a really interesting test that I'd never done before.

1.5 degrees of cant is definitely noticeable - 3 degrees is incredibly obvious and the idea that someone would pull the trigger with 5 degrees of cant on the rifle is beyond comprehension.

I can definitely say that I won't be spending money on bubble levels in the future.

I think the data hints at the math being fairly simple for a good estimate. It looks to me like for every 60 inches of boreline drop you have with your trajectory, 1 degree of cant will get you more than an inch of horizontal error at small cant angles, (zero to 5 degrees).

For a 30 Wolf at 1,000 yards, 1 degree of cant will get you about 4 inches of Horizontal error.

A 308 win. at 1,000 yards with one degree of cant will net you over 7.5" inches of horizontal error, and 2 degrees will get you more than 15 inches.

A 50 BMG will have more than 27 inches of horizontal error at 2,000 yards with 1 degree of scope cant.

It appears that at cant angles less than 3 degrees, any error on the vertical axis caused by cant is almost too small to measure, the majority of error is lost in the noise of the other small error sources.

Having participated in the test (but not having shot it due to unforseen problems), I had a couple of thoughts.

First, vertical error due to cant is irrelevant. Just go ahead a cross that line off your ballistic tables. Any cant angle large enough to create measureable vertical error will result in a huge POI shift horizontally. In other words, if you're canting so significantly that your shot will go 6 inches low, you needen't worry about it since it will be 4 FEET left of the target. Also, as is generally the case, SPEED WINS. Cant is more important if you're shooting something pokey and less important if you're shooting something smokey.

STL and S1, when you did the "return to level" test, were the targets, berms or any man-made feature visible in the field of view? I wonder how much of your ability to level the crosshairs was due to visual cues and how much was due to a physiological feel for the gravity vector. One of the places where I used to hunt elk in WY is a huge clearing that mostly slopes away from the shooter, however, there is a non trivial left to right slope as well. I can see how, if your field of view didn't include any horizontal or vertical references, you might have a tendency to visually align the crosshairs with the terrain. This could inadvertantly introduce some cant around the line of sight.

I propose that we test a shooter's ability to level the crosshairs with no visual cues (e.g. alignment against a white wall) with misleading cues (e.g. field of view zoomed into a sloping hill). I'd also like to test a shooter's ability to level the crosshairs when the shooter's body is in various awkward positions. Because if the primary contributor to crosshair leveling ability is the presence of visual cues, real-world terrain will make leveling difficult and could even CAUSE cant misalignments.

Also, before we toss out the idea of bubble levels entirely (which I'm currently inclined to do), does anybody know how precise a scope or rail mounted bubble level is? I just got out my 24" Home Depot level and I can see a perceptable shift in the position of the bubble when I put a 1/8" hex wrench under one end. I'm no math major but I think that correlates to precision better than 1 degree.

Anyway, just some thoughts.

Cheers,
nacroswnd

quote:

I propose that we test a shooter's ability to level the crosshairs with no visual cues (e.g. alignment against a white wall) with misleading cues (e.g. field of view zoomed into a sloping hill).

Well first off, I have yet to hunt the "Great White Wall". I am not sure how useful that would be....

Although I think your comments about varied and sometimes deceptive terrain would make for interesting and also useful test results. From a hunter's perspective, there is this curious phenominon about the legs of big game when standing on sloping terrain. The front leg always seems just about vertical unless they are drinking with their head below their feet. I guess they just do not have the strength to support all that meat with muscle tension the way Albert Pujouls stands at home plate. Seem to me vertical bone support is needed for them to stay on their feet in the long term. It is hard to imagine a place where there is Zero vertical reference. Living things have a posture that is key to balance, and anyone familiar with that animal will have very strong indicators about vertical and horizontal alignment. Now if you are just hunting rocks, all bets could be off.

You should try hunting The Great White Wall sometime. The trophies look pretty clean in your house.

When I was an EMT we had a fancy-schmancy ER doc come and tell us about some new medical device. It was way too complex and unreliable to be used in the field. One of my counterparts put it perfectly. "Look Dr. Fancypants, I'm sure it works great in the ER where you tested it. But you work in a place with air conditioning, level floors and good light. We don't want to try to use this improved nasal canula hanging by our feet in the middle of the night trying to work on some guy in a wadded up CRX half submerged in some river."

Also, you're probably right that nature isn't entirely devoid of accurate visual cues, (trees grow straight up etc). But it would be worth checking to see how accurate a pure feel for gravity technique is. I volunteer as a experimental candidate.

Cheers,
nacroswnd

I guess if you got really confused, you could just pick up a small rock and drop it, noting its vector as it falls to earth. If there are no small rocks around, simply tilt your head to the left side, and watch the last marble descend to earth from the void just west of your left eardrum, (provided you are shooting north of course...)

It is important to note that down from your jacket and/or dry grass in high winds will not work with this technique.

For ultra long range, I guess we are back to fundamentals. Shoot the flattest thing you can get your hands on, get the vertical out of your load, and learn to read the wind.


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Horizontal Error in inches with 2 degrees of cant.



Yards_______________600_____________1000___________1760______________2200



308 win. _____________4_______________14_________________________________
178 A-max
2700 fps.



50 Bmg.
750 A-max
2680 fps.____________3________________10____________40________________70



30 Wolf
210 VLD
3300 fps.____________2________________7_____________33________________65

Men,
I missed the device you used to "measure in 1deg increments of cant?
I personally use a bubble level on my 300 and perhaps I might do better if I knew of a better device.

KMB8--- The device is a large degree wheel that operates on the principal of gravity. Its resolution and repeatability will resolve .25 of a degree with no problem.

S1,
What's the name of that anti-cant device and is it mountable on the rifle.
As it has been pointed out and experienced, cant is a problem here, here.
Bubble levels are not reliable and have much too much deviation at these ranges.
For a carpenter layin out a 20 foot fence, ok.

The Degree wheel used came from the machine shop and is typically used for set ups. Much too large to be practical in the field, and too large to affix to the rifle.