spin drift

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by malcarjeb, Feb 16, 2008.

  1. malcarjeb

    malcarjeb Well-Known Member

    Messages:
    224
    Joined:
    Mar 12, 2006
    Could anyone tell me if the you need to allow a little for the spin of the bullet at 500 yards or longer? I was shooting today from 500 to 1000 yards and it was pretty still but i had to add a few clicks to keep it straight.
     
  2. Augustus

    Augustus Well-Known Member

    Messages:
    436
    Joined:
    Jan 6, 2008
    I have seen some lively debates concerning this topic and this may start another one you will probably see there are those who believe in it and correct for it and those that dont. Personally I believe at distances under 500 yds you wont notice it much. At 1000 it can be substantial, around 1 moa. The problem can be masked or compounded by how your scope is set up. I set my scopes up by using a rail type base, I then put a bubble level on it and mount the scope. Before you tighten the scope in the rings go downrange and set up a verticle line that you know is plumb. I use grey duct tape on a 4' by 6' piece of cardboard attached to a target frame. Put a plumb line from top to bottom then stretch the duct tape along the line. Now you can look through your scope and cant the rifle until the rail level indicates level, at the same time twist the scope in the rings until the verticle crosshair of the reticle is centered in the duct tape from top to bottom. The longer distance this is done the better. All my scopes have holdover type reticles so I dont have to worry if the reticle is canted inside the scope. If you are a clicker you need to make sure it tracks OK. Now and only now can you form an opinion about spin-drift. I notice about 1 moa at extended ranges. If shooting South at 1000 you will see around two inches added to that because of the earths rotation, shooting North about that much less. We are talking about right hand twists and the effects will vary due to differences in twist rates, velocity and bullet design. One of the icons in longrange shooting actually cants his scope to compensate for spin drift. If your scope is canted counter-clockwise it is compensating, if it is canted clockwise it will be making the drift appear to be worse. The best advice I can give you is to set your rig where the reticle is not canted one way or the other then go burn some rounds in perfectly calm conditions. This way you can make up your own mind.
     

  3. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,248
    Joined:
    Aug 10, 2003
    I think you got it backwards augustus.
    If you use holdover with your reticle, then a plumb vertical IS very important. Otherwise, if you dial in elevation with a std crosshair or dot reticle, the only thing that matters is a plumb turret.
    And of course tracking either way.

    Spin drift is NOT coriolis. Coriolis is known and calculated.
    Spin drift is not so predictable as it varies with your particular shooting system components(like bullet & twist & local air density). Spin drift is also not accounted for in any ballistic software I've seen.

    You will have to measure and account for it on your own.
    Or limit your distances(which is practical)
    Or shoot & shoot & shoot until finally hitting what you're aiming at(like so many others here).
     
  4. malcarjeb

    malcarjeb Well-Known Member

    Messages:
    224
    Joined:
    Mar 12, 2006
    I did use a level to make sure the scope is straight. At the 1000 yard mark I did notice about 1MOA that i had to adjust for and there was no wind to measure. I was shooting from north to south. I never really thought about the direction making a difference.
     
  5. Augustus

    Augustus Well-Known Member

    Messages:
    436
    Joined:
    Jan 6, 2008
    Mike, I did address two different things in the post, one was spin drift and the other was the effects of the rotation of the earth, spin drift seems to be around 1 moa at 1000 with the ones I shoot. At 1000 the effects of the earths rotation is around 2.5 inches. It will be added to the effects of spin drift when shooting South and taken away when shooting North. This is so small that it can rarely be seen due to being masked by other factors such as wind and errors in hold. On the subject of canted reticles. I certianly would not want to get in an argument but I think when you get a hold-over reticle verticle it will obviously stay that way as you raise it for come-ups. When you plumb a reticle there is no gaurantee that the intersection of the crosshairs are going to track perfectly up and down the plumb line. Since I have never been a clicker I have never taken a scope with an internal cant problem and tested it. I think what will happen is that after plumbing it on a plumbline and you begin to click vertically you will see the intersection start to drift off the line one way or the other and it will get progressively worse the further you go. Someone who knows from experience may want to kick in here as I would be interested in what they have to say.
     
  6. Rustystud

    Rustystud Well-Known Member

    Messages:
    431
    Joined:
    Dec 25, 2007
    I regularly shoot targets to 1000 yards. With proper spin to bullet weight ratio you will not be able to detect spin drift. If your bullet spin rate are improper your groups will be irratic from the get go. Also if you bullets go subsonic they will go irratic. I know of no top level shooter who say they can not differentiate spin drift and wind at 1000 yards. I have shot with Austrailians, New Zealanders, and South Africans and asked if they have noticed differences in groupings when they shoot north of the equator. Non have told me they could identify a significant grouping change from below and above the equator. We all know the gravatational pull is reversed on the other side of the equator..
    Rustystud
     
  7. James Jones

    James Jones Well-Known Member

    Messages:
    2,854
    Joined:
    Jul 1, 2002
    I have talked with some guys that shoot out pat 1000yds often and only a had full said that they could notice a differance in grouping due to spin drift. I was even asked by one fellow if it would be possibe to index the barrel curve to counter act the differance in spin drift , I'm sure it possible but would it be worth the effort? and I can't see how you could get accurate determination unless you were shooint in completly still conditions.
    Maybe some of the guys here that shoot out to 2000yds can chime in and tell what they see out past the 1 mile mark
     
  8. Augustus

    Augustus Well-Known Member

    Messages:
    436
    Joined:
    Jan 6, 2008
    I did not say that it was difficult to see the effects of spin drift, that is around 1 moa at extended ranges in my experiences, I said it was difficult to see the effects of the earths rotation due to it being so small. David Tubbs has went into great detail concerning both of these subjects on another site. He does believe that spin drift exists and should be compensated for. The bottom line is if you are shooting alot and you are not drifting right at extended ranges in calm conditions then dont worry about it, on the flip side If I dont correct for it I will hit about 1 moa right at 1000 and beyond. As stated when this subject came up, it is usually a volitile subject and you either believe it or you dont.
     
  9. Augustus

    Augustus Well-Known Member

    Messages:
    436
    Joined:
    Jan 6, 2008
    Rustystud, the Coriolis has nothing to do with which side of the equator you are on, it does not matter if you are 30 deg. N or 30 deg S. the effects are the same,it does matter which direction you fire the round from that location. The effects will be different at different latitudes, anyway this is mostly gee-whiz info because a shooter wont need to worry much about it until you get way out yonder.
     
  10. Shawn Carlock

    Shawn Carlock Sponsor

    Messages:
    1,895
    Joined:
    Jun 11, 2007
    I recently did a bunch of testing with diffferent calibers at distance to determine the effects of spindrift at the request of Nightforce scopes. I mostly confirmed what I already knew about spin drift.
    1. Spin drift exists in all calibers, some so slight as to not be practically detectable, others quite a bit.
    2. Spin drift will be effected differently in the same caliber by velocity, twist rate, bullet profile and atmo conditions.
    3. Spin drift is predictable after some testing, after determining the spin drift (this is not real easy) at a given distance like 1000 yards other distances can be predicted by entering it as a wind function. After determining 1000 yard drift I was able to predict 1500 + drift using the wind drift function and my 1000 yards info as a validation number. It will be a small number usually less than 1 mph of wind.
    4. The longer and higher BC a bullet the more suseptable it is to spin drift.
    5. Proper twist rates reduce spin drift but don't eliminate it.
    6. If spin drift is roughly 1 mph or less in most calibers few can estimate the wind that accurately. This doesnt mean that you should discount spin drift as it is, at distance one of the small compounding errors that can make a miss. I believe that these misses are more often interprited as wind estimate errors.
    7. Every single change in platform can effect spin drift differently, velocity, twist etc.
     
  11. Boss Hoss

    Boss Hoss Well-Known Member

    Messages:
    1,843
    Joined:
    Nov 10, 2005
    The algorithms that would be required to properly account for spin drift would be so complicated that accounting for the various types of rifling, air temperature, density etc that it would be a veritable science project but it could be done with enough money and timeā€¦

    Yes it is a factor in the world that I compete in.
     
  12. Augustus

    Augustus Well-Known Member

    Messages:
    436
    Joined:
    Jan 6, 2008
    Shawn, thanks for taking the time to share the info in on this, I just about went crazy when I first started shooting 1000+. I was shooting a 50 cal. with 750 A-maxs. The combo shot pretty well and would group decent but it would hit dead on at 100 yds, about 3 in right at 600 and the longer the distance the worse it got. I had never heard of spin drift at the time and I thought I must be canting the rifle or the reticle wasnt plumb or some other error I was committing. I bought a base level, got everything plumbed and had the same problem only a little worse. I then noticed the 300 ultra was doing about the same thing just not as bad. Yep, the 308 does it too. I discussed this with quite a few people and was told everything you can imagine. Most said it was caused by undetected wind, some said I just couldnt shoot, many went into great detail about how I wasnt getting a good cheek weld or I needed to curl my little toe on my left foot just prior to breaking the trigger. I began to think it was because I was born naked when I was a baby. Finally I came to the realization that something was causing the projectile to drift to the right. I would get up early in the morning and tie little white feathers from the posterior of a chicken all up and down the range, located where they could be observed while engaging the targets at various ranges. What I learned cost a young fortune, Shawn just got through telling everyone in less than a minute what took me a year, a barrel, and who knows how much money to find out. A pure target shooter will never figure this out, because he will go to the range, shoot a sighter, and correct for windage. The rest of the day will be spent compensating for changing conditions. This works great for their purposes but for someone needing to make first round hits on littlebitty things way out there, spin drift is definately part of the equation. I agree with the gentleman in the prior post, a program that could figure this out for every round loaded with every make, shape, or form bullet would take some doing; however any individual can sort this out without a lot of pain for the combination he is shooting. Do the numbers that Shawn posted and see if they are significant in your situation. If not, dont worry about it. If you are shooting at game 800+ you should know how much it affects you and compensate for it. By Shawns numbers it effects my 300 ultra about 7 1/2 inches at 1000. This is very close to what I have seen. That in itself is the difference in a clean kill and a good day gone bad.
     
  13. Shawn Carlock

    Shawn Carlock Sponsor

    Messages:
    1,895
    Joined:
    Jun 11, 2007
    While it might be possible to build a program to determine spin drift for a given combo, I think it would be nearly impossible to run all the possible combos available. While testing the 300 WM 190 gr there was a noticable difference in a 10 twist barrel vs a 9 twist. Also there was a subtle but consistance difference between 2 1-10 twist barrels of different rifling profiles. I believe that spin drift is an important factor at distance but truly requires some testing and practice to deal with. Once again even if we had a program to determine SD it would still be just a place to start field testing. Once you have the effective wind speed figured out to equate the SD, it has proven to be an accurate method of prediction in the testing I did from 1800-6600ft, 35-80 degrees, 30-80% humidity in various combinations and distances out to just over 1500 yards. In the Nightforce ballistics program I simply entered .85 mph right wind into the base chart and saved it. If I had an estimated wind of 6mph left, I entered 5.15 mph left. This provided for some truly outstanding first round hits on rocks of the 10-12" range at 1500+ yards. Without the SD compensation the would have mostly been close misses written off to slight wind estimation errors. It can drive you nuts but I really dig this stuff.
     
  14. Augustus

    Augustus Well-Known Member

    Messages:
    436
    Joined:
    Jan 6, 2008
    It never occurred to me to deal with this as a function of wind. This is gonna be GOOOOOOD.