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spin drift

 
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  #1  
Old 02-16-2008, 10:36 PM
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spin drift

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.
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Old 02-17-2008, 02:06 AM
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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.
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Old 02-17-2008, 08:21 AM
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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).
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  #4  
Old 02-17-2008, 09:19 AM
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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.
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  #5  
Old 02-17-2008, 11:48 AM
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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.
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  #6  
Old 02-17-2008, 12:48 PM
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Spin Drift

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
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  #7  
Old 02-17-2008, 02:05 PM
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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
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