SPIN DRIFT!....I'm such a dummy

[Bart B]

I've shot my .308 Palma rifle alternately at 1000 yards and 100 yards with the same zero for 1000 yards. One target at 1000 and another in front of me on the 900 yard line 100 yards away. My sight's elevation axis was in the true vertical as set using a spirit level on the sight. Alternately fired on each target for 10 shots at each one. Time of day was 5:30 AM in cool weather in dead calm conditions. There was no movement of the air seen at all. No movement of weeds or grass but I was watching stuff that's much lighter in weight that small vegetation and in the line of fire; it wasn't moving either.

Note that my .308 has an angular change of trajectory of about 1.8 degrees starting out at a departure angle of about .6 degrees and an angle of fall of about 1.2 degrees. Windage errors held at zero for both distance. Whatever spin drift there was didn't show up. If there is some that can be repeatedly observed, then so be it.

Those wanting to see some interesting stuff, checkout:

http://en.wikipedia.org/wiki/External_ballistics#Gyroscopic_drift_.28Spin_drift.29

 

[RDM416]

Bart, Thanks for the link. There are a lot of additional links at the end of the wikipedia link that I think will be very interesting and informative. I will dive into them this evening when I have a little reading time.

The first chart seems to pretty much confirm what I have seen in other publications, and on my ballistic programs. I am shooting a 338 cal with either 300 grain Bergers or SMKs at around 3000 fps. The chart in wikipedia shows 6.5" SD at 1000 yards. That corresponds to what I seem to be seeing in actual range results. I do certainly agree with you that there are many other factors involved and reading the wind is never an exact science. As you have pointed out, even when you think you have a "no wind" situation there is always some wind over a 1000 yard distance.

I have an engineering background and am starting to dig more into the science behind all this. Obviously you have a far greater understanding of this than I do, but I am trying to learn all I can. I do not mean to be "snarky" in my replys to you, I just have not understood why you dismiss spin drift as a variable that needs to be corrected for. I do understand now from your example that you are simply not seeing it with the example you stated in your previous post. In the end that is what it really comes down to. What we see on the range is what matters regardless of what the books or other shooters say. For me and the rifles and loads I typically shoot, I am seeing what "appears" to be spin drift at ranges approaching 1000 yards and beyond. When I dial in the correction my groups are more centered around my POA. You have me curious and I will continue to watch this and consider what other variables could be causing what I am seeing. But in the mean time, I am going to dial in what my ballistic program shows for spin drift.

 

[elkaholic]

Speaking of spin drift I've got kind of a strange one for you. As mentioned above in an earlier post I have my spin drift corrected at distance so that I will be slightly left at the closer ranges. With all of this talk of spin drift I re-checked my own spin drift correction at 1050 meters. I shot at first light with a maximum of 1/4 to 1/2 mile per hour winds. I had a flag on the target, at 750 meters and at 500 meters that I could watch before letting the shots go. All flags were down when I shot my 3 shot group that was 9 and 9/16". The group is still centered horizontally at this distance which is what I fully expected. The next morning I decided to shoot at around 740 meters in the same no wind conditions. Same thing, all flags down and shot 2 side by side that measured 1 and 1/2" apart. The thing is that the pair is 1.25MOA left of the bull. So, at 100 meters I am .5MOA left, 740 meters I am 1.25MOA left and at 1050 meters I am centered. This suggests to me that at least in my rifle that the spin drift seems to be an "arc" rather than a straight line value with the most drift taking place in the last 300 or so meters. In other words; if I'm .5MOA left at 100 to be centered and corrected at 1050, in theory I should be no more than .5 MOA left any distance between 100 and 1050. In fact the closer to 1050 the more right my point of impact should be moving correct?

I don't state this is fact because I don't know for sure but your theory on "ARC" makes some sense for this reason: Bullet rotation "spin" slows down at a much slower rate than forward momentum. Since the time of flight slows down between to points, but the spin doesn't (as much) it would make sense that the bullet would have more time to spin off course.
It is easy to see the affect of spin maintaining at distance by firing two identical bullets into gel using different twist rates at the same velocity. The greater the spin, the greater the wound channel is in diameter. i.e. more twist = more damage, all else being equal........Rich

 

[Bart B]

So, at 100 meters I am .5MOA left, 740 meters I am 1.25MOA left and at 1050 meters I am centered. This suggests to me that at least in my rifle that the spin drift seems to be an "arc" rather than a straight line value with the most drift taking place in the last 300 or so meters.

If you plot those numbers on graph paper, you'll see the bullet starts out to the left, then goes more left then back to the right to center about point of aim. That's not what spin drift curves do; they don't change direction. They're much like trajectory curves except horizontally and they're always in the same direction as the spin of the bullet. If it's .5 MOA left at 100, it'll never be more than that down range.

 

[Loner]

If you plot those numbers on graph paper, you'll see the bullet starts out to the left, then goes more left then back to the right to center about point of aim. That's not what spin drift curves do; they don't change direction. They're much like trajectory curves except horizontally and they're always in the same direction as the spin of the bullet. If it's .5 MOA left at 100, it'll never be more than that down range.

If the said bullet has 6" of drift @ 1000 it should be 3" left by the time it gets to 300 or
so as it is drifting back to the right shouldn't it? There has to be a horizontal arc if you
are shooting left and the bullet winds up poa.

 

[Tumbleweed]

If you plot those numbers on graph paper, you'll see the bullet starts out to the left, then goes more left then back to the right to center about point of aim. That's not what spin drift curves do; they don't change direction. They're much like trajectory curves except horizontally and they're always in the same direction as the spin of the bullet. If it's .5 MOA left at 100, it'll never be more than that down range.

That was my conclusion as well. I don't see how it could move any farther left than the original .5MOA left. Not sure why it's doing what it's doing but instead of worrying about it I will simply correct for it. Like someone else stated on here, all that really matters is knowing what your set up is doing at all ranges and simply adjusting for the things that we can't necessarily explain.

 

[Bart B]

I don't see how it could move any farther left than the original .5MOA left.

There's only four natural things that move a bullet off the barrel axis it was fired from.

One is an unbalanced bullet; centrifugal forces make it jump off barrel axis as it clears the muzzle.

Another is gravity; the same stuff that pulled that apple down to earth as Newton watched it.

Then spin drift; the gyroscopic forces that change the angle of attack the bullet has to the air it moves through that causes it to move horizontally through air. It's always to the right of a vertical plane through the bore axis for right hand twist barrels; to the left for left hand twist.

And finally, the medium the bullet moves through can move in all directions; it's the air being moved by wind.

I think you can deduce what caused the bullet to not have a constant curve to the right.

 

[Pdvdh]

The chart in wikipedia shows 6.5" SD at 1000 yards. That corresponds to what I seem to be seeing in actual range results.

I have an engineering background and am starting to dig more into the science behind all this.

When I dial in the correction my groups are more centered around my POA.

But in the mean time, I am going to dial in what my ballistic program shows for spin drift.

'Roger that' on all four of the above sentences. Except for the second sentence. The researching part is past-tense for me.

I'm experiencing about the same SD at 1000 yds. Between SD and Coriolis-caused drift (60.5 degrees North Latitude), I typically experience about 10" rightward bullet drift over 1000 yds. Which is very close to the predicted dope using LoadBase 3.0 ballistic software.

I noted this drift the very first time I moved back to 1000 yds about 3 years ago - before I owned any ballistic software. So I didn't experience the drift because I knew it was expected. Quite the contrary. AFTER experiencing this unexplained drift, I spent a lot of time researching the cause(s) of this rightward drift. Now I believe I understand it, and dope for it on all shots past ~5-600 yards.

 

[Bart B]

What I don't understand is why the competition shooting community seems to have such contempt for the LR hunting community. You BR guys will always shoot better groups. You get benches and rests, wind flags, sighters and fairly level terrain.

I am part of the competition community. I don't shoot BR. I don't use benches; I lay on the ground holding the rifle to my shoulder with my hands. Flags are only about 20% of what I watch; the other 80% is the air. I don't always get sighters.

I only have contempt for folks who think there's only one way to shoot small groups and make claims that contradict the laws of physics.

 

[paphil]

Spin and Coriolis Drift

That was a great link Bart, here is another by Brian Litz that also applies.

 

[vendetta333]

i just read this whole thread, and the article and it was fantastic! god i cant wait til i can afford to get into practical long range competition!!