Spin Drift and Baring surface

[Idaho Trecker]

I've done some reading on the theories/calculations on spin drift and one thing I haven't seen anything about that I thought about is whether the amount of baring surface has any affect on spin drift.
For instance if it was possible to have the same grain/style/BC... etc but different amount of baring surface and shooting same conditions from one rifle with one twist would you expect there to be more spin drift with the bullet with the longer baring surface?
Just curious what people think...

 

[royinidaho]

I"m thinking, though I know nothing about it, that baring surface length has little to do with it.

I'd think that things like barrel twist would have more affect but seeing as it is a constant for any given rifle it makes no difference.

I am not aware of any inputs to shooting solutions other than bullet length that may be a factor.

Interesting thread starter....

 

[jmitchell135]

I have been reading a lot on the topic too and I think you might be on to something.

I understand the rifling to cut grooves or leave indentions or impressions, however you want to see it, in the bearing surface of the bullet as it goes down the barrel. Say for some reason the bullet goes into the chamber crooked and one side gets a groove started further forward than the other side, then the bullet would have more friction on that side and start a wobble sooner than if it was evened out.

Now with more bearing surface you would have more to get an impression on. I would think it would have more effect on the flight of the bullet. Say causing more friction possibly even changing the BC of the bullet....?

I also understand the direction of wind to have a slight vertical effect on the bullet so with that in mind, the more surface area that is "gripping" the more it would be effected. Think about wider tires.

This is just my understanding as I have read and how it has been described to me from some pretty knowledgeable folks. I may have misinterpreted so don't burn me at the stake.

 

[Trever]

How would drive bands play into this?

 

[jmitchell135]

How would drive bands play into this?

Interesting to think about.... I don't know enough about it to have a good answer, but it would make the rifling grooves substantially smaller in the bullet. I am curious though how much the aerodynamics would go down by having the band on the bullet in the first place? The profile of a VLD would go from that of a lamborghini to that of a 1 ton duallie. Wide in the hips if you will.

 

[Trever]

Would it be correct to think that surface area, friction (texture), and twist rate would be the deciding factors. Maybe even velocity.

 

[Idaho Trecker]

All of your responses have me thinking about this more in depth. What got me thinking this was shooting my 338 ultra and going by the rule of thumb and using 1 moa to correct spin drift alone. I am shooting the 285 grain A Max and noticed how much more surface it has compared to that of the 250 and 300 grain berger vld. The surface of the 250 is much smaller than the 285 but the 300 is almost identical.

If only I had a way to shoot both bullets under all the same conditions and rifle specs to get an idea. I know there are so many different factors between the bullets it may be impossible to find a definite answer. I think for a test it would still be interesting to see how much difference there would be with spin drift between the 250 berger vs the 285 a max since they roughly have 0.2" difference between surface area.

Too much thinking

 

[royinidaho]

Too much thinking

Roger that!

Seem's to be a trait for me.....

 

[Mikecr]

Once you've noramlized BC, you've normalized spin drift from a given twist and distance.
You can't change bearing surface without changing BC, so that part(as separate) doesn't matter.
Same with shooting conditions, which affect BC.

 

[snowpro440]

Not really worth my time to sort that way of bearing surface and has not made a difference on paper . I think its more the conditions than anything and maybe a good shooting position that will have more affect on it.