Home Forums Hunting & Shooting Ask The Gunwriters gain twist barrels

Wind drift does not depend on the time of flight. It is a function of the velocity lost. That is why it is related to the
Ballistic Coefficient.
Good luck!

Thanks for bringing that up Hawkins. <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />

Hawkins,

Can you tell me how to get Sierra to correct their ballistic charts for me. The charts in the back of the Sierra loading manual show wind drift and muzzle velocity inversely related with the same bullet.

I am sure it must be a mistake on Sierra's part. Being a customer oriented company, I am sure they would reprint the tables correctly if only they knew the facts.

Mebbe our friends at Hornady will do that for us?

Indy,

The nail in the coffin--if there is one--for gain twist would be 1000 yd BR. The accuracy requirements of 1K BR are significantly greater than the accuracy requirements for Camp Perry and High Power style shooting.

Idaho Shooter; Bullet drop depends on the time of flight.
Bullet drift depends on the velocity lost. As drag depends on velocity there would be a correlation. sort of like saying the
trees moving make the wind blow.
Good luck!

I, for one, would love to see some pressure testing of gain twist barrels to see the difference, if there is any.

+1
That would be interesting...

More precisely, according to the Krtizinger formula, wind drift is proportional to the extra flight time induced by air resistance and velocity loss.

Wind drift = wind speed perpendicular to the trajectory x (flight time - distance/muzzle velocity)

Naturally, the flight time will depend on the ballistic coefficient of the bullet, which is itself dependant on is aerodynamics and its sectionnal density.

According the the RSI Shooting Lab, a 6.5mm 120 grain projectile that starts at 3200 fps will experience 14.6" of deflection at 500 yards, with a 10 mph crosswind. At 2800 fps, it will experience 17.71" of deflection at the same distance.

What in the world are you guys saying about wind drift,BC, and velocity? All I know is that the same bullet driven at higher velocity drifts less at a given yardage than when started off slower.

OK, Denton beat me to the obvious. Hornady agrees.

As does Sierra.

Wind drift is dependent upon time of flight which is determined by muzzle velocity and ballistic coefficient. Both factor highly into the determination.

I just spent a bit more time with the reloading manuals and the wind drift formula.

Wind drift is a product of the side wind velocity vector multiplied by lag time. Lag time is actual time of flight minus a theoretical value derived by muzzle velocity multiplied by distance traveled. (as Deersmeller already told us)

Lag time is dependent upon muzzle velocity as well as BC, therefor so is wind drift.

I still do not quite agree with the theory in totality, and perhaps
Denton can help us out here as he is a much better mathematician than I. But Sierra assures us the formula is close enough for the purposes we use it for.

If this formula was exactly theoretically correct, then there would be no drift correction required for a projectile upon which thrust was continually applied so that there was no deceleration in flight and therefor zero lagtime.

But we know that rockets in flight and aircraft do have to make corrections for wind deflection, so obviously a wind drift formula totally dependent upon lag time is only an approximation.

For the most part, wind drift does not come about from wind blowing the projectile off course. It comes about because the bullet faces the direction of least resistance, which is the vector sum of the crosswind and the "wind" that the bullet experiences by moving through the air. The bullet is not facing exactly forward. The drag vector acting on the bullet is directly out the tail of the bullet, so it has a huge vector component backward, toward the rifle, and a small component across the path of the trajectory. It is this small component of drag that is the main actor in pulling the bullet off trajectory.

So, to figure deflection, you have to know the BC (affects the total amount of drag, and both components), speed of the bullet as a function of position, perform an octuple integration, and perform the lambda gamma transform.

OK, I lied about the octuple integration and the lambda gamma transform thing. But it is still a lot of math.

I just use RSI.

Riemann, Stieltjes, Henstock, Denjoy, Perron, Daniell, or Lebesgue?

mathman

ugh!
Those names bring back bad memories!!

A couple of those are somewhat esoteric even among mathematicians. Which ones did you run into?

mathman

Riemann sums? and have heard of Stieltjes... none of the others.
Those in a class called transport phenomena.
Dealt with heat transfer, mass transport, and diffusion phenomena.
Rather heavy on theory!!

Hi Steve,

I don't know if you're aquainted with Mic McPherson either in person or thru his writings but he has written several aritcles about the potential benefits about gain-twist barrels (the ones he wrote about were not as radical as mine) and included in the articles what he thought produced the benefical effects.

Mic was instrumental in getting the barrels for me but I don't know why he never followed those early articles with further tests as I think that was about the time he became involved with the design of his new case.

He's really a fascinating fellow and the veritable Mother Lode when it comes to reloading and rifles.