Why is "Overstabilized" bad

Let's put some perspective into energy losses due to spin....

From the acclaimed "Rifle Accuracy Facts", by H.R.Vaughn, published by Precision Shooting, chapter 10:
"The effect of bullet afterbody lenght on drag is very slight. The main effect at high velocity is the shape of the nose as a result of the high pressure actin on the nose. Most of the rest of the drag is caused by the low pressure in the wake acting on the base. The ratio of head drag to base drag at 3000 fps is 2 or 3 to 1. At lower velocities the base pressure becomes more important relative tothe head or form drag and this is why a boattail becomes more effective at lower velocities of Mach numbers. The skin friction drag developed in the boundary layer is less than 5 percent because of the laminar boundary layer. The effect of rifling marks on drag have been tested and found to be small. The reason is that the rifling depht is only 2 or 3 mils and is buried in the boundary layer. Also, the rifling marks are tangent to the free stream velocity until the bullet slows down. The spin rate slows to some extent but not nearly as fast as the flight velocity."
 
Variations caused by a bullet's true shape are nearly negligible. - Kernel
Click to expand...

You're almost there, Kernel. You're right about long bullets being heavier than short, but it's not just about weight (mass); it's moment of inertia that's important. MOI along with RPM is the key.

Case in pont about bullet shape: when Ned Roberts designed his .257 Roberts, he chose a twist too slow to stabilize a 120-grain spitzer. He discovered, though, that it would stabilize a 120-grain round nose, but he didn't know why. Here's why: moment of inertia is a measure of mass distribution. A long, skinny, pointed bullet has more mass concentrated near its longitudinal centerline. A square-ended wadcutter of the same weight has its mass distributed further from its centerline. A round nose is more like the wadcutter in mass distribution. Stability depends on MOI and RPM; at his .257's velocity, Roberts' twist was just fast enough to stabilize the round nose with its greater MOI, but too slow to stabilize the spitzer with is lesser MOI. So bullet shape can play a critical role.

My mascot was a bowlegged cowboy, by the way. ;)
 
Hey Don, which is heavier, a 55gr bullet made of feathers or a 55gr bullet made of lead? hee, hee. Trick question! The correct answer is a 55gr bullet made of depleted Uranium. -snicker-

Jeff has got it figured out. Moment of Inertia - in the back of my mind I knew bullet shape played some small part in stabilization but couldn't remember what it was. Compared to RPM MOI is like a whale swallowing a Tic-Tac... it has an effect but it's pretty small.

Don, you said "Sectional Density varies greatly depending on the bullets shape and composition..."

Not true. All bullets of the same weight and diameter have the same sectional density. SD has got nothing to do with length. SD is just weight divided by cross sectional area. My feather, lead, and Uranium bullets would all three have the same SD. Maybe you were thinking of Aspect Ratio which is length divided by sectional area.

And another thing.... of course tracers are longer, they've got a figgin' hollow base. And because they're hollow and loose mass as they fly (tracer material burns up) they're intrinsically unstable. Heck, they want to fall over just standing still. That's why they need a high twist, not because they're a teeny tiny bit longer than a conventional bullet. Talk about apples to oranges.

What we need is a "Unified Field Theory" for bullet dynamics. Something that ties together interior and exterior ballistics, all the variables in obscure Greek letters, inergral signs, and dx/dt notations. Something that will answer all the questions. Something to intimidate the twits that never took Differential Equations. Something that I can program into the C compiler on my Palm Vx Palmpilot (the slide rule of the 21st Century).

I remember reading an article in Precision Shooting a few years back that this Ph.D. type wrote. He explained how when setting up a rifle range it should always point true east or west. Never north or south because the earth's rotation causes the target to drift away from the path of the bullet, it was called "terrestrial progression" or something. WTF!? He had the math to prove that at 600 yds it would disperse your group by 1/4" or something. Then he went on to briefly touch on many of the other lesser known phenomena that effect bullets like the gravitational force of the moon, Corollas Effect, and about 37 other things.

Nasser's the one who's got me wigged out. I always just assumed that the RPM of the projectile was directly proportional to the velocity. Now I learn that RPM is nearly constant even as velocity falls off. What's that equation look like?

Now, don't argue with me anymore or I'll have your BetaMax copy of TRON confiscated, all your HP calculators destroyed, and your membership in MENSA revoked! -- Kernel
 
We are getting close here

MOI, mass distribution and bullet length are all important factors in stabilization. What is lacking here is the reason stabilization is necessary. Fired in a vacuum, NO twist is necessary for the bullet sees no force other than gravity. Atmospheric drag and its variabilty is why we need rifling twist for standard bullets. If we put the CG ahead of the CP by finning al la 120mm smoothbore MBT rounds, rifling would be history.

Length of the bullet is key for it determines the overturning moment arm when a disturbance is encountered. The moment arm is the lenght from the extreme tip to the CG, ususally located along the center axis behind the center of pressure for spitzer types. What happens as Roberts et al discovered, is making the profile a round nose is the CG moves forward AND the overall length decreases while the CG moves forward slightly. This enables a 120 gr RN .257" bullet to be stabilized by a 1:14" twist in warmer air. Take it below 0 F where the density of the air is higher and the overturning moment increases due to the instability force alone. Same thing was noted for the ogiginal M16 twist of 1:14"
 
"You can read all the books in the library; after three days the cheese still turns green." (Old Polish proverb referring to the inevitibility of natural law.)

The Greenhill Formulae is outdated in theory. However, in the real world, one can't change rate of twist whenever you change bullets; and one doesn't confine oneself to just one bullet per caliber.

So.....

One has one of two options:

Primus: Build your rifle barrel to accomodate the longest bullet (requiring the fastest twist), or...

Secundus: Determine the twist of your rifle barrel and limit yourself to the heaviest bullet you can stabilize.

By the way, moderate to much overspin is better than a little underspin.
 
so, i had this 55 grain bullet made out of dark star material.
but i lost it...
i ordered a replacement one from the Vogons, but they miss shipped
one made the volume of a lead 55 grain bullet and the UPS guy can not pick it up.

They are compaining that they have broken 3 forklifts trying to get it on the truck!

;)

Some day soon, a program like this will be able to explore most of the shooting factors
http://www.shooterready.com/rangegame.html

dZ
 
Military bullets tend to be overstabilized because...

... you are expected to hit things that may be behind light cover or hiding in the bushes or the tall grass; overstabilized bullets tend to go straight through, whereas barely stabilized bullets will be deflected much more.

RPM is determined by twist and muzzle velocity; as the velocity decreases, the distance covered per rotation decreases also (RPM remains the same).
 
A few heavier vs longer posts have missed their mark. Remember that when Remington originally introduced their .244 with a 1-12" twist it failed to stabilize 100 gr spitzers like Winchester's 1-10" 243 did. Speer introduced a heavier but shorter 105gr roundnose that allowed .244 owners to use their guns on bambi.

A round ball stabilizes at the slowest twist, a longer projectile doesn't.

Tom
 
Note that the Army teams "overstabalize" their 80 grain Sierras when they shoot at 1,000 yards with their AR-15s (they use a 1 in 7 twist). For "across the course" 600 yard matches, they use 1 in 8 twist with the same bullet. "overstabilizing" only seems to cause problems up close.
 
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