Can't find my spin drift or coriolis.

Discussion in 'Long Range Hunting & Shooting' started by lisagrantb, Aug 28, 2009.

  1. lisagrantb

    lisagrantb Well-Known Member

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    Ok, here goes! Sighted dead on at 100 yds when I move to 1000 I was expecting 7-9 inches of drift to the right, I actually got 0 inches.

    Now the technical stuf: 260 Remington, 140 grain Berger VLD @ 2860 FPS. Rifle shoots a constant .5MOA @ 1000 yds or better.

    I have checked and rechecked the barrel, scope and reticle for plum ness, I’ve even machined a sliding double V block with a bubble level on top to make sure everything was aligned. I am 100% sure the rifle/scope is correct. According to the computer I should get .6moa right spin drift and .2moa right coriolis deflection for 31deg lat. This is a total of .8moa drift to the right. With dead calm wind I constantly get 0 to .1moa drift to the right, I do have a level on the rifle so I know I’m not canting the gun. I have rechecked the zero at 100 yds numerous times and it is always dead on. Not having to compensate for SD/CE is good but I am wondering why it’s not there, any thoughts on this.
     
  2. phorwath

    phorwath Well-Known Member

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    Dunno. I confirmed 9" of rightward drift at North Latitude 60.5 degrees last weekend. 300 Win Mag with 10-twist barrel. 210 Berger VLDs.

    A 1-2 mph crosswind is all you'd need to cause that much drift at 1000 yds, so it's vital to test in wind-free conditions.
     

  3. AJ Peacock

    AJ Peacock Well-Known Member

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    If you have a place where you can shoot both directions, you can see if an unseen wind (up above the ground where the bullet is flying) is affecting you.

    I would re-verify my zero at a longer distance. Knowing that your horizontal is perfect (within 2/10moa) or so is very difficult. See if a group at 200-300 yds is still perfect left/right.

    AJ
     
    Last edited: Aug 28, 2009
  4. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Coriolis is affected by bullet path (direction) as well as lat. Shooting East-West or vise versa will cancel coriolis effect. North - South travel will give you the greatest effect.

    -MR
     
  5. Michael Eichele

    Michael Eichele Well-Known Member

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    The only way to cancel CE is to shoot at the equator. Sorry, not trying to argue. Northern hemeshere is always to the right regardless of direction of fire. The farther north you go, the greater the effects. Southern hemeshere is the opposite. Always left. The farther south you go, the greater the effects. The direction fired in either hemeshere will affect the vertical component of CE but the horozontal is not measurabley different.
     
  6. britz

    britz Well-Known Member

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    I was always lead to believe that the coriolis effect had to do with flight time, the speed at which the earth is moving and the direction relative to the shooters line of sight (Azimuth).

    I edited my post after looking through some other posts. As it turns out.. my physics isn't what it used to be lol! I don't understand why, but I guess your right. LOL! I'll try to wrap my brain around it some other time ;)
     
    Last edited: Aug 29, 2009
  7. Topshot

    Topshot Well-Known Member

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    Maybe you have a left twist barrel? If so your spin drift would be to the left.
     
  8. lisagrantb

    lisagrantb Well-Known Member

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    No it’s the standard right 1-8 twist. I have detailed records from just about every shot from that gun and when I go back I can reverse engineer every shot. With known range (900yds or longer). Wind speed and direction, point of aim, point of impact and firing solution from the computer for elevation and windage with coriolis and spin drift included , it always comes out no greater than a .1moa right drift for both spin drift and coriolis combined. Thanks for the help on this but it shoots where it shoots and I cant change that.
     
  9. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Michael, I believe you are probably right and I stand corrected after researching this a little which I should have done before typing. Although I haven't found any articles that go into a technical explantion yet as to the effect on external ballisitcs. So I think there may still be some question as to how a moving objects track will affect its CE. Still researching it although what I've found seems to back up what you have said.

    The reason for my statement is that coriolis effect is essentially the movement of an object (target) on the surface of the Earth as the surface of the Earth moves while spinning on its axis. In My minds eye it would seem that movement of an object on a spinning surface of the Earth would be more pronounced going N-S or S-N. Tracking E-W or W-E ther would be no relative movement of the target because both shooter and target are both moving in the same direction along the same linear path.

    At the N Pole every direction is South and vise versa at the S pole, so that backs up my theory in that case. At the Equator, the Earth's surface is moving its fastest, a little more than 1000 mph so it puzzles me why there would be no CE at the Equator, shooting S-N?

    Puzzled and still researching.

    -MR
     
  10. phorwath

    phorwath Well-Known Member

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    MR,

    I researched this about one year ago. Michael is correct, and there's nothing I could add to clarify his post. It was interesting and educational researching the topic, so I'd continue until you've had enough.

    lisagrantb,

    8-10 inches at 1000 yards could be caused by a slightly misaligned scope, a slight cant, a slight wind, and slightly less than perfection in any number of areas of execution. I'd run with whatever your target proves to be correct, which will take into consideration all of the potential affecting factors. You don't see any spin drift or coriolis, then run with that. They are affecting your bullet flight each and every shot, regardless of whether or not you can account for them. They're somehow getting lost in the noise of the minutia of factors that can slightly affect POA versus POI at 1000 yds.

    I've learned some things don't matter much until you move past the 600 yd range. Then there's any number of things that begin taking a toll on the theory versus the reality.
     
  11. Topshot

    Topshot Well-Known Member

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    One more thing you could try.

    Repeat your experiment on a calm day. This time include a target at 600 yds.
    Shoot one shot at 100yds, one at 600yds and one at 1000yds. Wait till your barrel cools then fire one at 1000yds, one at 600yds and one back at 100yds.
    Repeat this process until you have fired say five shots at each target.
    Then plot the centre point of each group and measure it relative to the aiming mark. This process should remove some of the variables like variations in the strength of any slight breeze.

    If you are getting any drift at all then this experiment should pick it up.

    Drift is not liner , so take the horizontal difference between the 600yrd group and the 1000yrd group, add 50% and that will be roughly the amount of drift you are getting in total.
     
  12. MontanaRifleman

    MontanaRifleman Well-Known Member

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    This sounds to me like a good problem to have if where it shoots is zero drift. Be happy.

    -MR
     
  13. lisagrantb

    lisagrantb Well-Known Member

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    Thanks guys for all the input. I think I will try the 100/600/1000 yard test at a different range. This will answer two questions at once, yours and the one from PSW. I contacted Precision Shooters Workbench ( the ballistic program I use) and he attributed it to subtle prevailing wind from the right caused by the terrain on the range I usually use, and yes, I’m glad it shoots to POA and not the other way.
     
  14. lisagrantb

    lisagrantb Well-Known Member

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    [FONT=TimesNewRoman,Bold]
    I was sent this interesting article a few days ago and I think it may have hit on my problem. I'm going to build a jig to give me the accuracy I need, about 36 seconds or better. When I get it built and tested I will post the results.



    CHARLEY ROBERTSON'S
    [/FONT][FONT=TimesNewRoman,Bold]
    SCORE HIGH GUNSMITHING
    [/FONT][FONT=TimesNewRoman,Bold]
    Precision Scope Mounting: By, Charley Robertson
    [/FONT]
    Long range rifle shooting is more popular than ever. This is evident by the growing number of products marketed specifically for long
    range shooting. Today every scope manufacturer has a product line of long range scopes enabling the average rifleman to accomplish
    what only a few years ago could only be accomplished by a few very elite, highly trained individuals with equipment so expensive
    only a government agency could afford to posses it. These scopes, fitted with target knobs and ranging reticles can only be an asset to
    your rifle if they are properly mounted. Mounting a long range tactical scope sounds on the surface to be an easy enough task but in
    reality nothing can be further from the truth. Having built hundreds of custom rifles and mounted thousands of scopes, I did not give
    the job enough ​
    [FONT=TimesNewRoman,Bold]serious [/FONT]analytical attention until I started shooting long range tactical matches and building long range tactical rifles
    for some of the country’s top shooters. While shooting at various matches, I noticed several inconsistencies among very talented
    shooters. A typical conversation between three competitors all shooting 308s at 800 yards might go as follows:

    [FONT=TimesNewRoman,Italic]
    (#1) “Man there must be more wind out there than I can see. I held ¾ MIL right and I still hit left.
    (#2) “No kidding? I only held ½ a MIL right. I hit, but it was almost too much.”
    (#3)“Really, I held right on and smacked them all right in the middle.”​
    [/FONT]
    How can three talented shooters have such a wide range of results? I thought it was condition changes that occur from shooter to
    shooter that could not be seen. Or, maybe it’s the way each scope is set up on the rifle. What if the scope is mounted on a rifle in such
    a way that the vertical cross hair is not in perfect plumb to the rifle? Or a problem even harder to detect, what if when turning the
    target knobs to dial in the correct amount of elevation, the center of the cross hairs moving up and down are not exactly perfectly
    plumb? How far off do things have to be before it matters? This concept of keeping everything perfectly straight is not new, I’ve
    been doing that for years. I vise up on a rifle and place a good Starrett bench level on the receiver rails and make sure the cross hairs
    follow a line I have drawn on my shop wall. I needed these questions answered, so on a no wind day (for New Mexico that is a 3 to 5
    mph wind) I experimented. I had two rifles chambered for the new 6.5 Creedmoor. Our shop built both rifles and they were identical
    set ups both having their scopes mounted in the conventional manner. There was a minor difference between the two in that one had a
    Leupold Mark 4, 6.5x20 FFP with a TMR reticule and the other had a Leupold Mark 4, 8.5x25 FFP with a TMR reticule. Both rifles
    had a good 100 yard zero. Shooting at a 12”x10” steel plate at 755 yards with a 3 mph wind from 3 o’clock, using a center hold, The
    shot from the first rifle hit just off of the left edge exactly where it should have and the shot from the second rifle hit ¾ of a mil to the
    right. At the range, I loosened the rings and rotated the scope on the rifle that was shooting too far right, so minutely that it was
    almost imperceptible in fact I wasn’t sure I even rotated it. Shooting again at 755 yards the point of impact moved about ½ a MIL left,
    and hit just off of the right edge. With a 3 mph wind from 3 o’clock using a center hold, it should have been just off of the left edge.
    I rotated the scope again in the same direction, and it moved another ¾ MIL in the same direction. Now it is shooting too far left. The
    other rifle that I left alone did not change point of impact proving that conditions were consistent. Simple fact, scope mounting
    matters, a lot! My next question was how can I make sure I attain the “perfect scope mounting job”. I can’t take every long rang rifle
    to the shooting range and mount and remount scopes until everything works right. I MUST have a reliable method to perfectly mount
    a scope in the shop and know without question that it will function 100% correctly in the field. I set forth to design and implement a
    system that would do just that.
    The new system must secure the barreled action (with the scope mounted) in such a way that the centerline of the scope and the
    centerline of the rifle’s bore are both exactly in the same vertical plane. Then, dial the elevation knob through it’s entire range while
    looking through the scope making sure the cross hair’s center never leaves a perfectly vertical plumb line. Rigidity is the key. It is
    absolutely imperative that operating the target turrets over and over again causes no detectable movement in the assembly. To make a
    long story short, we designed a carriage assembly very similar to that of a rail gun. That carriage rigidly holds the barreled action and
    scope perfectly plumb enabling the operator to simultaneously operate the elevation turret through its complete range and view its
    movement over the perfectly plumb line. I obtained a perfectly plumb vertical line using a laser level on my neighbors building 35
    yards from the shop. This eventually evolved into a grid with MOA and MIL marks accurately drawn on it so that scopes could be
    evaluated as far as their true movements, but that is another story.
    Using the new system the scope on the problem rifle was rotated in the rings so that it accurately tracked on the laser line. Another
    trip to the range. I use a software program on a PDA called ​
    [FONT=TimesNewRoman,Italic]Field Firing Solutions [/FONT]from www.precisionworkbench.com . After
    verifying a 100 yard zero the target at 755 yards was engaged. With a 7 mph wind from 135 degrees my software called for and I
    dialed in, 17 ¾ MOA of elevation and a hold of .4 MIL right. I got a center hit with both rifles. This was a great success.

    [FONT=TimesNewRoman,Bold]
    Conclusion:​
    [/FONT]
    I have always taken great care mounting scopes. The
    techniques I had been using were perfectly correct in
    theory. I simply wasn’t getting the degree of accuracy
    required. Originally both rifles went through the same
    process. I got lucky on one, not so lucky on the other. The
    difference between right and wrong was imperceptible
    using our old system in the shop environment. I could not
    get satisfactory results until I built a very elaborate fixture
    that could rigidly hold a barreled action with the scope
    mounted in such a way that the vertical centerline of the
    bore and the vertical centerline of the scope had no more
    variation than one thousandth (.001) of an inch in six inches.
    Some rifles are just good shooters and they make it easy for a good shooter to shine. The performance of some rifles defies logic, and
    makes it impossible for the shooter to excel. The shooter questions their ability to dope wind. They get frustrated and filled with self
    doubt when in reality they might just need to have their scope properly mounted.​