Why is "Overstabilized" bad

B

Blue Duck357

New member
I understand a bullet needs a certain twist rate to properly stabilize, but if the rates too fast it overstabilizes and accuracy is again less than optimum.

So what exactly is going on with a bullet that is "too stabilized" that makes accuracy poor. It seems you would want it as stable as possible?

Thanks, Blue Duck
 
Here's a nice URL for you:

http://www.povn.com/~4n6/fig15.htm

fig15.gif



They're saying that over stabilized projectiles will remain pointing in the direction they left the barrel, even though its direction has changed. So while the bullet is traveling downward at the end of its flight path, the bullet tip is still pointing upward.
 
looks like the ballistic path on my paintball rifle!
:)
Consider the graphic an overstatement, but indeed that is the concept.

in reality the curve of a fired round is pretty flat
a six inch drop in 100 yards is just kinda hard to fit on 15 inch monitor
;)

dZ
 
Nice graphics.

My first guess was going to be that faster twist rifling strips more jacket material and fouls more than slower twist rifling.

Tom
 
OK, I am probably way off on this, but I'll give it a shot anyway (ha,ha).

I can think of 3 reasons why you would not want a bullet twist rate to be too high:

1. You may exceed the tensile strength of the jacketing material from centrigufal forces, causing the bullet to literally fly apart in its flight path

2. You waste too much energy in twist that is not otherwise directed to velocity; keep in mind that a moving object has a certain amount of energy and the higher the twist rate, the more energy is being diverted to this motion

3. Most rifle bullets are designed for hunting animals or wounding people (for battlefield-type cartridges). You ideally want a twist rate that is fast enough to stabilize the bullet in flight, but slow enough that it becomes easily destabilized when it enters soft tissue, thus "tumbling" and fragmenting to create a wider wound channel.

Any and all, feel free to correct me if I am wrong. I am still a novice shooter trying to learn.
 
Gee, and I always thought rifle bullets went straight because those little devils who mess up accuracy couldn't hold on to the spinning bullet.

Seriously, Destructo6, that is a great site and certainly answers all the questions.

Jim
 
Hi Rock_jock, let's see....

1. You may destroy the bullet (or casuse jacket/core separation) because of too much centrifugal force.

2. A fast twist is hard on the rifling, but muzzle velocity is the same.

3. For military use "...you ideally want a twist rate that is fast enough to stabilize the bullet in flight, but slow enough that it becomes easily destabilized when it enters soft tissue, thus tumbling and fragmenting to create a wider wound channel". This may be right (even though the military use a very fast 1:7 nowadays), but I think that this effect also depends a lot on bullet construction.

4. As explained before, overstabilized bullets don't "track" well, but this is only important at long ranges.

5. Another very important factor: upon firing the bullet is forced to rotate around its geometric axis when travelling inside the barrel, but all bullets are slightly imbalaced, and when flying outside they rotate around their true center of gravity (CG).
Due to the small CG offset a tangential velocity component is produced, and manifests itself as a lateral drift velocity when the bullet exits the bore. The distance that the bullet will deflect due to this drift is proportional to the CG offset, the muzzle velocity (the real one, discounting the small increment due to muzzle blast), and the time of flight to the target, and inversely proportional to the twist rate (muzzle velocity and twist rate determine the lateral drift velocity).
An overstabilized bullet exagerates this lateral drif effect; a 1:8 twist will produce a 50% larger deflection due to bullet CG imbalance than a 1:12 twist (provided of course that 1:12 is enough).
 
According to "The Black Rifle"

the 1/7 Nato twist "overstabilizes" the 5.56 mm SS109 projo causing high torque levels on the jacket. The result is, on impact with a fleshy medium, the bullet breaks in half at the cannelure, causing two separate wound paths plus the collateral damage from stray fragments. Ain't engineering swell?
 
ALL 223 bullets at high speed break...

...regardless of barrel twist. Check the info regarding 223 bullet performance at the Olympic and Bushmaster sites, and go to:
http://207.181.246.106/johns/terminal.htm
to see the 223 terminal balistics on flesh, compared to other rounds. This break-up is almost entirely dependant on velocity, and in FMJ militaray bullets (55 and 62gr.) happens up to aprox. 200m.
 
Ruben,

Its good to see a fellow engineer on the board. Two comments on your post:

1. For the same powder charge and the same barrel characteristics (e.g., materials, design), a bullet fired from a rifle with a 1:7 twist should have a lower velocity both at the muzzle and at a given. This effect may be negligible, but I am certain it would be the case because the charge imparts a certain amount of energy to the bullet, which is converted to energy to move the bullet forward, and rotational kinetic energy to cause spin.

2. You state "The distance that the bullet will deflect due to this drift is proportional to the CG offset....and inversely proportional to the twist rate". You then state "An overstabilized bullet exagerates this lateral drift effect; a 1:8 twist will produce a 50% larger deflection due to bullet CG imbalance than a 1:12 twist"

These statements are imcompatible. I see your point about the CG and spin, but this means a higher deflection is PROPORTIONAL to a higher twist rate. I assue of course that by higher twist rate you mean a lower ratio, not higher, no?
 
Hi Rock,

1. The kinetic coeffient of friction is very low, and once you swaged the bullet into the grooves and started spinning it the energy loss due to friction is very small. This is why it may take 28" or more to achieve maximum muzzle velocity in a normal rifle caliber, but it would take a few feet more to start decreasing that velocity.

2. You are right, I did not express myself very well, the deflection is directly proportional to the twist rate.
 
Hi Rock,

1. The kinetic coeffient of friction is very low, and once you swaged the bullet into the grooves and started spinning it the energy loss due to friction is very small. This is why it may take 28" or more to achieve maximum muzzle velocity in a normal rifle caliber, but it would take a few feet more to start decreasing that velocity.

2. You are right, I did not express myself very well, the deflection is directly proportional to the twist rate.

If you are an engineer and like ballistics and the theory of rifle accuracy, you'll love "Rifle Accuracy Facts", by a real rocket scientist, H.R.Vaughn, available at sinclairintl.com.
 
Ruben,

I am not speaking of the energy losses due to friction, but rather the energy expended by the bullet DURING flight to maintain spin, expressed as the rotational kinetic energy. This is still an energy loss, and should translate into a reduction in forward momentum, thus a lower velocity.

BTW, thanks for the link.
 
(I love it when the Pocket Protector crowd gets a thread going.)

All good info. Let me add... twist rate and muzzle velocity determine the bullet's rpm when it leaves the barrel. It's a small but important point that barrel twist rate itself doesn't stabilize a bullet - rpm does - a function of twist rate and velocity. Heavy bullets need a faster twist barrel, not because they are heavy, but because they are slow. A fast twist heavy bullet will have a rpm comparable to a slow twist light bullet. Maybe this is obvious to everyone but it took me years to realize this fundamental fact. So the question when determining optimum twist rate for a given cartridge isn't "Are you gonna shoot light bullets or heavy bullets?" it should be "What's the expected muzzle velocity?"

The problem that comes about when you shoot thin jacketed bullets in a fast twist barrel is the bullet will fly apart due to internal stresses caused by the high rotational velocity. This is what happened when the guys with the 1:7 ARs started shooting thin skin varmint and match bullets in the mid '80s. They couldn't figure out why they couldn't get on paper. The bullets were literally disintegrating in thin air before they would get to the target. Not a problem with steel core military bullets. -- Kernel
 
The friction losses due to rotation are negligible. For example, after 1000 yards the velocity of a 175gr. 308 HPBT bullet is about 40% of its muzzle velocity, but spin rate is almost the same. This is because in flight the bullet is sorrounded by a boundary layer, (a thin layer of slowly moving air, due to air viscosity, that is several times thicker than the depth of the rifling marks) and this eliminates most of the lateral friction.
 
It's RPM that's key

Don,

First let me say your easy grasp of the technical jargon demonstrates an understanding of physics, aerodynamics, and engineering that could only be obtained thru years of diligent post graduate study at a major university.... one of the good ones... with lots of big buildings and books.... and a goofy mascot.... stuff like that.

But I gotta disagree on the length thing. Length's really got nothing to do with it other than the fact that longer bullets are heavier and shorter bullets are lighter. In Greenhill's formula length is a stand-in for weight. Think about it. IMO Greenhill used bullet length for one simple reason - it's easier to measure, all you need is a caliper. To measure weight you need a delicate, bulky, and expensive scale, it's gotta be calibrated, etc. Greenhill understood that within any caliber the relationship between length and weight is directly proportional. Variations caused by a bullet's true shape are nearly negligible.

Greenhill's formula works fine within a practical range but is less effective at the extremes. For example, could a 1:14 barrel stabilize a 80gr .22 cal bullet? Heck yeah! In fact it might be the optimal rate if the bullet had a muzzle velocity of 4800 fps. Shoot 30 grainers from 1:7 barrel? Sure, could be ideal if they're moving out at 1200 fps. You get the idea. All the variables are different but the RPMs are equivalent.

It sums up like this: Long bullets are heavy, heavy bullets are slow, slow bullets need a faster rifling twist to achieve the necessary RPM so as to not be understabilized. The flip side: Short bullets are light, light bullets are fast, fast bullets need a slower twist to achieve the necessary RPM so as to not be overstabilized. It's RPM that's key. -- Kernel



[Edited by Kernel on 02-01-2001 at 10:02 AM]
 
RPM! Of course - the light bulb just went off. It makes sense now. Thanks.

Now, another question.

Does all this mean that the tracer rounds had a lower velocity than the FMJ rounds? Does that make sense? Or is the idea that the twist rate went to 1:7 in order to handle the longer (heavier?) tracer rounds just balogna?

Waiting for further enlightenment.
 
Lenght and bullet shape ARE important...

The Greenhill formula is outdated and not very accurate, if you want to play and see for yourself the effects of bullet shape and lenght in its BC, go to:
http://www.lascruces.com/~jbm/ballistics/drag/drag.html
Imput the values for a .308" 180 RN bullet, and compare with a SBT bullet of the same weight; after that you can play and design your own super low drag bullets.
An initial stability factor of about 1.5 is ussually desired.
 
The kinetic coeffient of friction is very low, and once you swaged the bullet into the grooves and started spinning it the energy loss due to friction is very small. This is why it may take 28" or more to achieve maximum muzzle velocity in a normal rifle caliber, but it would take a few feet more to start decreasing that velocity.
Click to expand...

It's no energy lost that's the problem, it's heat generated that is. An already hot bullet gets ever-hotter as it spins because not only are you pushing air across the bullet, but you're actually giving it a bit more contact that a purely straight-flying bullet.

Lift is also a factor...
 
You must log in or register to reply here.
Back
Top