Wind X twist

L

Liseo

Member
Hi

I posted this question in other forum, without any response. May be a dumb question?


Suppose two identical rifles firing two same projectiles with the same speed and with exactly the same accuracy at a distance of 100 meters with no wind. The only difference is that one have 1/14 tw and other 1/8 tw. The difference in RPM will affect the precision more on one than another, if you have wind?

Just curious to know.
 
I guess I would have to counter your assumption that the 1:14 and 1:8 would be equally accurate at 100 with no wind. Other things being equal, the 1:14 will be more accurate than the 1:8. My idea is that it is less sensitive to minor imperfections in the bullet and is generally "easier" on the bullet.

One would use the 1:8 to stabilize a longer bullet with a higher ballistic coefficient. That bullet would be less sensitive to wind than the stubbier bullet that we tend to shoot in a 1:14.

Go to this page: http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi and compare the wind drift of a 65 gr 6mm with BC of .269 like one would shoot in the 1:14 and a 108 BT match that one would shoot in the 1:8 that has a BC of .511.

-- Greg J.
 
liseo

You forgot to tell us what the bullets are. Let's say they are 6mm, for example. Are they 60 grain FB or, are they 115 grain BT ??? And, while were at it, what's the velocity??

Makes a big difference.

I'd have to say that wind would not affect the accuracy but it sure would affect the precision.

Ray
 
Last edited by a moderator:
Maybe I was´t clear (or my english isn´t good enough)

I know that the heavier bullet will deflect less with wind. But if the bullets are the same ?
And that the light weight bullet have more accuracy potential when fired in slow twist barrel. But my question is ...with same bullet, one in 1/14 tw and other in 1/8 tw with same weight ,accuracy and speed without wind, when the wind is blowing, the bullet with higher rpm will
deflect the same or less?
 
Liseo,

Let's thin this down some. One, accuarcy would 99.999% of the time will never be the same because of the twist rate varriance so who cares. It seems to me, the real question is, will a bullet fired with the same accuracy, at different rpms, at differently in the wind. The rule of thumb is yes. BUT which one will be more accurate will probably have to be broken down into whether you are talking about a left to right wind or a right to left wind. See....the biggest difference in rpm and accuracy has to do with which dirrection the wind is blowing in. But again, it's only rule of thumb because there are always an instance where it doesn't play out and that is why we have percentages of the time.

Typically, a higher rpm bullet will drift more and print a larger group in a left to right wind (that is assuming a right hand twist barrel), and the oppisite is normally true in a right to left wind. This also plays out in a head or tail wind, normally a slower bullet will print a better group in a head wind and the oppisite in a tailwind. It kind of has to do with how the bullet wants to "ride" the wind.

I think this is the point that you were trying to reach. Of course, some will disagree with this but having dicussed this with many top shooters, they all seem to be aligned with this average as a rule of thumb. But they all will test their loads in the different conditions on that specific range to conclude what is really going on because factors of the range come into play.

Hope I didn't make to big a mess of this.

Hovis
 
According to the JBM ballistics link that I gave you....All other things being equal, the faster twist would deflect slightly more.
 
OK, full stop here....... this reply is to the original poster and in no way references any other answers.......

My answer applies ONLY to two identical bullets at identical velocities but one twisting 1:14 and one twisting 1:8

My opinion is that the faster rotation on the 1:8 bullet will result in a larger nose offset due to greater equilibrium yaw. Equilibrium yaw is driven by gravity and the rate of decrease of rotational velocity. Faster rotation will result in larger nutants or a greater precession force. Therefore, while wind drift will be very slightly increased the most observable effect will be that that the VERTICAL offset will be dramatically increased. In other words, the faster rotating bullets will print left/HIGHER and right/LOWER than the slower rotating ones (for a rh twist barrel.)


(Actually, this is imprecise.... the nutants themselves are SMALLER but much faster which drives the tip wider. No that ain't right either, the nose has TWO nods going on in a circular fashion and the LARGER nod circle is wider????......ANYways, you can look it up, the angle of repose is greater for "overspun" bullets).. .. .. .. .. .. But they never fly "nose high" like the goofs over at Rupprecht_Nennstiel try to convince us ;)


you might have to actually read a book to find an answer beyond mine.



al
 
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