bullet drop and scope leveling

[Bart B]

[ QUOTE ]
The person whom I talked to at Lilja (I cannto remember the name) told me that drilling a hole 34 inches deep and being on center on both ends was near to impossible.

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Something's wrong. Obermeyer and Kreiger have been gun drilling blanks that size with less than a couple hundredths of an inch error at either end. A gun drill automatically centers itself at the center of rotation by the shape of the tool; a long and well established method used by many top barrel makers who get excellent results.

Of course, Lilja had to supply several barrels to a couple of folks on the US Olympic team before they got one that would shoot well. I wonder why...... Maybe Lilja hasn't perfected their gun drilling methods.

 

[goodgrouper]

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I can attest to the fact that the coriolis effect of the earth spinning at different speeds (about 500 yards per second at the equator) depending on latitude is zero.

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I would disagree with this statement. It may be a small amount but it is still there. And the farther you shoot, the more it shows up. I have shot enough at 2000 yards plus to get some calm enough days that there was no other explanation for the phenomenon than for the "C" effect.

Another interesting point was that at 2k, every bullet that went left of the gong usually also shot high, and every bullet that went right of the gong went low.

At what points in the ocean were you shooting from the ships and did you notice a change in Coriolis as you got farther from the Equator?

But I believe that the original poster's problem (if he did truly level his scope properly) was as JBM stated. I have witnessed gyroscopic drift in every bullet fired to one degree or another and it is much more of a factor than Coriolis. If Coriolis makes 1" difference then Gyro makes 9". These are not the real amounts of course, just an example to show the magnitude in each.

If you are leaving your rifle zeroed at 1k for left-right movement and it is always on, then you have "zeroed" the "G" drift out and it may seem non apparent but it is still there although it may be in a small amount. You have simply just corrected for it.

 

[Bart B]

Rifle shooters have observed the phenomena of bullets going high-left and low-right for a century or more. If a left-hand twist is used it'll be low-left and high-right. It's most predominant with smallbore 22 rimfire rifles at 100 yards but gets less at shorter ranges. Some left-handed smallbore competitors cant their rifles about 35 degrees to the right so making windage only adjustments will compensate correctly in both axes. Centerfire rifle shooters have noticed it starting at about 800 yards and getting a bit worse at 1000 when some ammo types are used.

It's caused by cross winds putting different pressure under and over the bullet. A clockwise spinning bullet's surface pulls a wind from the right under it putting more pressure at that point so it moves left and up. The opposite happens with a wind from the left. Smallbore prone shooters will hold towards 4:15-o'clock when wind from the right picks up. Highpower shooters will hold off at 3:15-o'clock for a right wind pick up; the effect is much less than with 22 rimfire at 100 or 50 yards.

As for the coriolis effect on rifle bullets through 1000 yards, it may well exist. But it's so darned small and unimportant that none of the top highpower long range folks I know care about it nor get concerned about correcting for it. I once fired 30 rounds alternating between a 100- and 1000-yard target at 5 AM when the air was so calm and clear I could see bullet holes in the black at 1000 yards. The 15-shot groups at 100 and 1000 yards were centered for windage but the close one was some 40-odd inches high and straight up (measured with plumb line) from the aiming mark. Whatever drift effect there was didn't matter; it wasn't visible.

I don't remember much detail about gun aiming computers' corrections for latitude from my experiences in the late '50s. When I changed latitude, bearing and range values running tests on those marvelous mechanical monsters the gun orders in the horizontal plane also changed. How much was for precessional drift and how much was for the earth's rotation now escapes me. One can read about it by going to:
http://www.navweaps.com/index_tech/tech-056.htm
then scrolling down to the section "Correcting for the Coriolis Effect." The greater the range the bigger the correction was. More information or large caliber ballistics including the projectile's drift caused by gyroscopic precession or Magnus Effect can be seen at
http://www.eugeneleeslover.com/USNAVY/CHAPTER-17-A.htm

 

[jwp475]

I tend to agree with Bart on this as I have not experienced any drift horizontaly out to 1000 yards on calm (no wind) days.

 

[Chawlston]

I agree with the above statement. I routinely shoot up to 978 yards down a 30 foot wide opening through 60 foot tall pine trees that act as a very good wind brake.

I always zero my gun dead nuts right on at 400 yards and then check it on successive days to verify zero. We have even shot 5-shot groups over several days (ie two shots today, two tomorrow and one on the third day that were .5" or better. Afer the 400 yard zero is obtained and confirmed on three different days, we progressively move the drop board out in 100 yard increments to derive drop tables for the particular cartrige. Additionally, I have placed 3/4" maskng tape level across the top of the board and another 90 degress vertical to use as an aiming point.

We have found that the higher BC bullets tend to track closer to the vertical aiming line (ie lesser drift) while the lesser BC bullets travel further away (ie more drift) from the line. In fact during some of our testing last year, we were actually hitting the masking tape at the 900 yard mark when holding dead on. Finally, the bullet drift in minimal wind conditions was so slight with higher BC bullets, anything over a 2" drift was considered a bad shot.

We only shoot to derive the drop tables when it is cloudy or before the sun rays hit the ground causing mirage.

In short, using wind flags with the pine trees blocking the wind and shooting at times when mirage has not started has yeilded some very consistent shooting results. Our most accurate bullets were the Lost River bullets seated with the ogives .020" to .050" away from the rifling. Many times the two shots at each range, the holes were touching all the way out to the 800 yard line. We never got the 900 yard shots to touch. Too bad the Lost River bullets (both poly and metal tipped) do not expand. We found this out the hard way when deer were hit and knocked down only to get up and run off and later killed with healed-over Lost River bullet wounds.

Anyway, we did not see much drift due to spin and we were shooting 8 twist barrels.

James

 

[Brown Dog]

We did coriolis to death a while back; can't summon up the will to rehash it here so here's a link:

http://longrangehunting.com/ubbthreads/s...=true#Post29447


Bart,
Drift (or spindrift) is one of the few 'weird and wonderful' ballistic effects observable and measurable at 1000yds...the others (coriolis etc) are all at play, but not measurable. Drift absolutely is.

[ QUOTE ]
It's not yaw, but precession caused by the bullet's tip continuously pointing down as its spin axis stays parallel to the trajectory path. As the bullet noses over, its right-hand twist causes it to change its direction to, or drift to the right.

[/ QUOTE ]

...actually, projectiles fly slightly nose up in relation to their trajectory; this means that air resistance will try to lift the projectile's nose. Because the projectile is a powerfully spinning gyroscope it reacts to this upward pressure by yawing slightly to the right (it's just something gyroscopes do), with this new yaw, the changed direction of air resistance causes the nose to move down (again, because of the weird way gyroscopes react to forces)...then it will move left...then it will move up ....and so on round and round.
This creates an effect known as 'nutational rotation' ..a more complex movement than I can describe without drawing a picture.
....anyway, the net effect of all this movement is that (clockwise spun) projectiles average a 'nose high and right' attitude in relation to the line of their trajectory. This 'average orientation' of the projectile known as the yaw of repose. ...it should be obvious why a 'high and right' yaw of repose (also known as 'equilibrium yaw') will cause increased air pressure on the left side of the projectile and thus cause it to drift to the right.

...I think you'll find the values quoted by JBM are the average accepted 'norms'.....take a peek through a direct fire antitank gun sight next time you have a chance; their reticles are actually marked to account for drift...even at relatively short range /ubbthreads/images/graemlins/smile.gif.

 

[Michael Eichele]

[ QUOTE ]
I once fired 30 rounds alternating between a 100- and 1000-yard target at 5 AM when the air was so calm and clear I could see bullet holes in the black at 1000 yards. The 15-shot groups at 100 and 1000 yards were centered for windage but the close one was some 40-odd inches high and straight up (measured with plumb line) from the aiming mark. Whatever drift effect there was didn't matter; it wasn't visible.

[/ QUOTE ]

Sometimes if a shooters scope is not quite level with the rifle and happens to be in the right direction as you click or use hold overs for that matter it compensates for the spin drift leading one to believe that it doesnt exist. I assure you that this is a scientifec fact that spin drift occurs. Whether or not ones equipment is set up correct or not is another story. Personaly I have a right handed twist and know what my drift values are through tests and have my scope ever so slightly off of perfectly plumb. As I use clicks or hold overs it takes the guess work out of holding off to the left. My rifle is level and the level mounted to my scope reads level, but the scope is not quite level.

If you have never experianced spin drift and you were shooting in a DEAD still air enviornment you were none other than lucky to have screwed up mounting your scope and did something right. /ubbthreads/images/graemlins/wink.gif

 

[Chawlston]

The topic originators' issue is 8-10 inches drift at 800 yards. That is not solely spin drift and he has something else attibuting to the bullet drift. I know it exists, but not in the 8-10" value at 800 yards. Let's try to help him solve his problem.

James

 

[Michael Eichele]

8-10" is possible at 800 yards although that much at that range isnt that common. If its not the sole problem it could be 80% of it.

I guess we should ask him how sure he is that his scopes are 100% level.

So how sure are you that your scopes are 100% level?

 

[Chawlston]

There are too many variables in this debacleto say it is 80% of it without a lot more information. We never saw 8" horizontal dispersion in over a two hundred shots over the entire testing period last fall prior to the Elk hunt in Colorado. I shoot a lot and just cannot fathom it being that prevalent by itself. Anyway, unless you eliminate everything else, you cannot be sure that it is due to bullet spin.

James

 

[Michael Eichele]

Just because you didnt see 8" doesnt meen everybody else wont. Like I said 8-10" at 800 yards is possible but not common. I also never said that was his problem. I did say that it was possible. You are right, we dont have enough info from him. I am trying to make a point to those who say spin drift doesnt exist.

 

[goodgrouper]

Meichele,

You are exactly correct. Spin drift is a real ballistic principle and is not made up from fairy tales. The military along with countless other ballisticians have noted and described this very thing. The only way you could have a bullet not drift itself over is if you built a rifle range in a vacuum!

It is funny how people like to debate known facts about well established ballistic principles. This isn't anything new, it is ballistics 101.

The reason why some folks may not notice it is because it is <font color="red"> different in magnitude for every rifle.</font> SOme show more than others, but THEY ALL HAVE SOME!

To quote from R. Rinker in his book:
"There is no practical method of calculating drift from factors such as velocity, range, etc. Various bullet characteristics that cannot accurately be predicted are involved. Consequently, a direct measurement during testing is the only certain method for determining the drift. This evaluation is common with military ammo, but is not usually done with sporting cartridges and bullets."
pg 105



As for the distance the military found the 30-06 to drift, it was 13" at 1000 yards and less at closer distances. In other words, it still was evident at 800 yards and so forth.
Rinker states:

"For ranges below 1000 yards, it is suitable to use the approximate relationship that the drift is proportional to the square of the time of flight, or to the third power of the range. For example, at 500 yards the drift will be approx. 1/8 that at 1k or 1 1/2 inches. At 300 yards it will be about 1/30 that at 1000 yards or about 3/8 inches. While it is excessive at ranges well over 1000 yards, it is small enough to be of no concern at normal ranges. The Springfield .30-06 rifle's rear sight apertureis mounted on a slide that is inclined and also slanted to the left. As the sight is raised higher for longer range shooting, the sight also moves a slight amount to the left to compensate for drift."
Pg 105 bottom paragraph


So you can see, gyroscopic drift effects every bullet every shot and at every range. You simply zero your scope for drop as well as windage at the given range you intend to shoot and that is why it sometimes goes unnoticed.

Given that the poster's scope was leveled properly, this phenomenon is very suspect of the amount of drift he is seeing.

 

[goodgrouper]

[ QUOTE ]
Bart,
Drift (or spindrift) is one of the few 'weird and wonderful' ballistic effects observable and measurable at 1000yds...the others (coriolis etc) are all at play, but not measurable. Drift absolutely is.


Quote:
--------------------------------------------------------------------------------

It's not yaw, but precession caused by the bullet's tip continuously pointing down as its spin axis stays parallel to the trajectory path. As the bullet noses over, its right-hand twist causes it to change its direction to, or drift to the right.


--------------------------------------------------------------------------------



...actually, projectiles fly slightly nose up in relation to their trajectory; this means that air resistance will try to lift the projectile's nose. Because the projectile is a powerfully spinning gyroscope it reacts to this upward pressure by yawing slightly to the right (it's just something gyroscopes do), with this new yaw, the changed direction of air resistance causes the nose to move down (again, because of the weird way gyroscopes react to forces)...then it will move left...then it will move up ....and so on round and round.
This creates an effect known as 'nutational rotation' ..a more complex movement than I can describe without drawing a picture.
....anyway, the net effect of all this movement is that (clockwise spun) projectiles average a 'nose high and right' attitude in relation to the line of their trajectory. This 'average orientation' of the projectile known as the yaw of repose. ...it should be obvious why a 'high and right' yaw of repose (also known as 'equilibrium yaw') will cause increased air pressure on the left side of the projectile and thus cause it to drift to the right.

...I think you'll find the values quoted by JBM are the average accepted 'norms'.....take a peek through a direct fire antitank gun sight next time you have a chance; their reticles are actually marked to account for drift...even at relatively short range .




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I just read this post and Brown Dog, you are also exactly correct. Many people of bygone eras or "oldschool" way of thinking believed that the drift occured because the bullet was "rolling on air". Modern test have proven this wrong and the drift is caused by the bullet's nose being slightly above the trajectory curve causing the "yaw of repose" which steers the bullet to the right from a right hand twist as you mentioned so perfectly. In short, the air pressure differences caused by a rotating or spinning object give a bullet a push to the nose on the left side. /ubbthreads/images/graemlins/wink.gif
Very well put.

 

[Brown Dog]

Thank you GG; I managed to stay away from: "the nose rosettes in troichoidal motion about the trajectory" /ubbthreads/images/graemlins/smile.gif

...I do enjoy the bursts of ballistics nerditry here, gives me a chance to blow the cobwebs off stuff I did on a year-long course almost 10 years ago. I notice the last burst of nerditry I linked to was around 2 years ago! Tempus fugit /ubbthreads/images/graemlins/frown.gif !

Right....let's get into Magnus... /ubbthreads/images/graemlins/grin.gif