40Rod,
" there is not one just one velocity that will stabilise with most bullet twist combinations there are two or sometimes three velocities where accuracy will be good. you can see where accuracy is good. The best illustration that I can give is that when group size is plotted it is not on an x,y axis but but on a sine wave. "
Allow me to disagree - a stable bullet is stable, there's no different velocities where a bullet will be come unstable  and then stable again:  that's another thing entirely, and only related to accuracy - see als C. Audette's Ladder test, and the theory behind it.  IOW, more to do with barrel whip, and the point of exit of the bullet during barrel whip.  And in such a way, it is quite possible indeed to find several sweet spots.

I think the conversation has inadvertently conflated two independent concepts.  The first being stability and the second being velocity-related "sweet spots."

Typical schuetzen velocities range from approximately 1300 fps to 1500 fps.  Within this narrow velocity spread, the primary determinant of stability is twist rate.  If the twist rate is too slow to stabilize a given bullet, it is all but impossible to compensate by raising velocity.

Sweet Spots are an entirely different phenomenon.  Starting from 1300 fps and raising velocity in small increments, it is very probable that there will be several velocities where group size will shrink.  Some call these velocities "nodes."

In my experience, the sweet spot associated with the highest reasonable velocity will give the best overall performance over the broadest range of conditions.

JackHughs 

Dr Tom my uncle served on the South Dakota alias Battleship X during WW2. said those 16 inch projectiles looked like Volkswagons flying through the air. Had some very interesting stories and some very hard times.

SORRY Fellows now Back To Twist Rates

J.Louis

MartiniBelgian wrote on Oct 31^st, 2012 at 1:35pm:


Not to get off topic, but when considering barrel vibrations, don't forget that the barrel will do a lot more than move up and down.  Oscillating systems tend to have movement in all degrees of freedom.  i.e. the barrel will move up/down, left/right, and to some degree in torsion.

Chris.

OK, so there MUST be an APP for it!

Plug in type and SN of rifle, load, wind direction and rotation of the earth and it spits out elevation and windage corrections for each shot!

I'll jump in with my opinion, for what it's worth.

The stability of a bullet is slightly affected by the wind.  And, it depends on the direction it is coming from.  A side wind will have less effect than a head or quartering wind will.

The twist does have an effect on stability, as noted before, you can either understabilize or overstabilize, although I think it is nearly impossible to overstabilize at ranges over 300 yards.  At shorter distances, overstabilization can be a problem.  J Louis shoots 200 yards, his opinion and observations are valid for that range.  I shoot at longer range, and have no place to shoot 200, so what I observe is at ranges to 1000 yards, or at 100 yards only.  My long range observations are after the bullet has "gone to sleep", and so overstabilization is not a factor.

Understabilization produces a yaw in the bullet itself, where the bullet wobbles, or tips, about a center of rotation.  Overstabilization produces the large spiral path downrange, as J. Louis noted the bullets do not follow a straight path, rather a long lazy spiral.  The long spiral of overstabilization is not subject to wind induced instability, the wobble yaw of understabilization or misbalanced bullet is.

For an understabilized bullet in a quartering or headwind, the bullet yaws and presents more surface area, so the wind slows it more, drifts it more, and if gusting, can destabilize it more.  This is because the destabilizing wind force has more apparent surface area to act on.

The faster the twist, the more aerodynamic lift on the bullet.  That effect is the same as air on an airplane wing.  The faster the airspeed, the more the lift.  Incresing bullet spin increases the apparent airspeed over the surface.

My opinion is that the gyroscopic stabilization of the bullet by twist helps the bullet be more stable in wind.  More is better.  Marginally twist guns don't shoot well at long range in the wind.  Overly fast twists shoot better in high wind.
dave

Yesterday at the range I tried to help a guy shooting the Lee 230 gr .30 300AAC bullet in a .308 Win. 10" twist Ruger. At 50 yards, some bullets made oval holes, up to ~ 1300 fps. Over 1300 fps the bullets made bullet shaped holes, when they hit the target.
(With my 314299, 21/IMR4227 .308 loads, he shot 5 into ~3/4" at 50 yards, so it ain't the gun.)
Magnus forces?
joe b.

For us mere mortals, understanding all the jargon of yaw, magnus effect, center of pressure is fun to talk about but for most, we'll never be able to apply it. Understanding what affects wind and spin drift do to our bullet flight is essential.  Both groups are shooting through the transition which really can create havoc with our bullets.  John and Jack measure their wind drift in tenths while us LR shooters like Dave and I measure it in feet.  Tenths and feet are both critical in shooting a good score
Imagine that you're spotting and your partner just shot an X at 1000 yds.  Wind has been steady at 5mph from 10 o'clock and then you get a shift  to 10mph at 2 o'clock.  Now what are you going to tell him to do on his next shot?  You give him the corrections and he looks up at you and says, "huh".
Fishtailing winds from 8 to 4 are the hardest for me to grasp.  This is what is fun and makes us comeback to try to improve.  Bob