Shiloh Rifle Forums

What is the formula for stabilizing a bullet? What all comes into play?

* Greenhill twist formula
*. Powley calculator for stability
Browse Internet for each

* Greenhill twist formula
*. Powley calculator for stability
Browse Internet for each

The Greenhill stuff will get you started. One of the best expositions I have found on the subject is "Modern Exterior Ballistics" by Robert McCoy. It leads you through the history of exterior ballistics, beginning in the 19th century, then introduces modern concepts. There are a lot of factors involving momentum and aerodynamics that keep bullets from going where we want them to go. He lays everything out so that you can get an understanding of the physics of what is going on, even if you don't want to get into the differential equations that provide a rigorous mathematical description. This was the Bible of the late Dan Theodore.

Another great resource is the century-old work of Franklin W Mann, "The Bullets Flight". He arrived at a rather thorough understanding of the flight of bullets, though the science of aerodynamics was in its infancy at the time. It is widely available, and should be studied by EVERYONE who is interested in shooting lead bullets (and who ain't?).

CHRIS

With most rifles in the past I have been that geek guy that crunches numbers on a ballistic program to come up with the ideal bullet for a certain application.
I love reading published data on bullets and even went to great lengths to find the BC of big ole lead bullets, but when I started shooting BPCR I wanted things simpler.

I ask successful shooters what they use and then see if it works in my rifle.

RCBS made a big ole 40 cal BPCR bullet mold that dropped bullets at about 430 grains and it looked way cool...but neither of my 40-65s liked it.
Lyman's 410663 doesn't look that cool and has thin grease grooves but shoots well in both our guns.

I have light for cal. bullet molds for my 45-70 that shoot not so good at 200 meters, but will be just perfect on a hog ( like the herd I saw this morning) within 50 yards of my deer stand.

Watching the Mile guys crunch numbers is very interesting to me but I like big ole slow lead slugs right now.

The various twist rate charts available on the internet are handy things to look at and will certainly give you a good starting point.
But that's about where things start running off into left field. You can get away with big ol long bullets in midrange and silhouette, and they can be useful in long range,,, until the wind comes up.
I am of the opinion that going with a bullet just a little short of what should work is the better option. Then test the bullet you decide on at the furthest range you will shoot it in a good stiff wind. Keep an eye on the vertical and dirt diggers and sailers, and left right bullet strikes that don't agree with the wind indicators.

Coltsmoke. I have simple formula. bore .450X3=1.35x1.35=1.8225''. twist 18x.07=1.26. then devide 1.8225 by 1.26 answer will be length of bullet. Vic from the land down under.

I remember something about how to determine what the nose length of the bullet should be and something about 55% of the caliber. Must be the result of CRS.

I always wonder if anyone really knows for sure what instability looks like while the bullet is still in the air. Do bullets in flight that begin to wobble always result in a tumble or do some wobble part way out then right themselves on axis rotation ?
I remember reading about the Seagirt trials where they recovered round nose 500 grain bullets at two miles that were still point on and from the penetration did not seem to be in much of a destabilized mode at impact.
I believe it was a .45 cal rifle shooting about 100 grains of powder and the twist was 1-19 if I remember correctly.

Mathematical calculation and advanced theory shudder at the approach of reality.

Mike,

Are you thinking of the Sandy Hook tests with the .45-70 in 1879 maybe? I think they tested the 405 grain 500 grain bullets out 2 miles.

It's not uncommon to see a bullet that has lost instability going to the target looking like it's riding the handrail on a circular staircase. I suppose at some point it will tumble.

I have noticed and talked with others on occasion that have group shot certain loads at progressive yardage and have found the groups open up at a given point and then shrink back down father out. If it occurred one time it means nothing other than wibbles compensating for wobbles but if it keeps repeating than something else is happening.
I once read a good article some where along the line( I can't remember where) about cones of dispersion created by certain bullets and velocities that seemed to be directly related . I have often wondered if each bullet repeats this pattern of positioning in a shot string making up that combinations cone of dispersion. If so than could not the shot count be sight adjusted for to point advantage ?

I sure would like Bruce B's input about now! I miss him already!

Vaguely remember hearing/reading the same thing about cones of dispersion.
May be wrong but the reporter was of the opinion that round barrels were less affected.