External Ballistics for Beginners

J

JEC

Guest
I have little understanding of physics but somethings just don't make sense, and I know there are those on this thread that can explain.

I was told that it is possible to stabilize bullets that are "marginal" in terms of rotational stability, by increasing velocity. I think the theory is that increasing velocity will "spin" the bullet faster.

This doesn't make sense to me as I can not imagine that bullet rotation can increase after leaving the barrel, and while inside the barrel... the bullet rotation must be dictated by the twist, regardless of velocity. So, a bullet that is rotating one turn for every 10" as it travels through the barrel...must start to slow its rate of rotation as soon as it exits the barrel as it encounters the forces of friction and gravity.

Increasing velocity will decrease the time it takes a bullet to get to the target, but the bullet still has to plow through the same density and distance of air.

What have I missed? Please be gentle!
 
Jec

Yes, the faster you push a bullet in terms of fps, the faster it will rotate. Remember, the spin is in so many turns in a given length. Cover this 'length" in a shorter time, and the bullet rpm increases.

You can see this with a cleaning rod. Take a good tight patch, and slowly push the cleaning rod through the bore while looking at the rod turn. Then, push it through fast. Notice the rod spins at a higher rpm. Do not get RPM confused with twist in a given length.

The bullet is spinning as fast as it ever will the instant it clears the bore. After that, all of the laws of thermo dynamics take over. While the turns per linear foot remain constant, the RPM will slow as the bullet sheds velocity.

I actually had a shooter at Tomball tell me he believed that a bullet actually sped upalittle when it exited the bore because it was freeof the barrel friction. It is also free of the force that was propelling it in the first place. That is what he was missing.

As for hoping to stabilize a bullet that is unstable by pushing it faster;, I have never been able to do it. Many times, it takes more velocity to achieve the needed RPM than the case has capacity to deliver.

Some of the math whizzes on this Forum can give you the figures for RPM as it relates to velocity and barrel twist. I hope Randy Robinett of BIB bullets chimes in. He knows all of this stuff.......jackie
 
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I cannot give a scientific formula to support the results that velocity can improve stability/accuracy of a bullet out of a specific twist..BUT...try dealing with sub-sonic loads and you will get a real eye-opening experience..:eek:
A rifle chambered for a .357 magnum or 30BR..as examples..shoot like "poop" at 100 yds until the velocity is brought up from 1088 fps to say 1600+ fps..
By increasing velocity the bullet spends less time in atmospheric conditions.

Just my take on it...and by the way "I hate sub-sonic loads"
 
Thanks Jackie...I understand that RPM increases with velocity, but wouldn't the number of rotations the bullet makes over a given distance be the same regardless of velocity...it would just get there in less time?
For example:
In a 1-12 twist, a bullet exiting the barrel into a vacum makes one rotation every foot it moves so it would turn 100 times in 100 ft. Launched at 1fps, it would turn 100 times in the 100 seconds it takes to get to the target. If launched at 2fps it would still turn 100 times but would reach the target in 50 seconds. Since the bullet makes the same number of rotations, regardless of velocity, gyroscopic stability should remain constant.
Now, if we put that same bullet in "air", it is affected by friction for 100 feet, regardless of whether it it is travelling at 1fps or 2fps. Unless increased velocity decreases friction, rotational decelleration should be relative?
Come on engineers...what's happening?
 
RPM Formula

Here is the formula for determining bullet RPM:

MV x (12/twist rate in inches) x 60 = RPM

or

RPM = MV x 720/twist rate

1 in 10 twist at 3000 fps = 216,000 rpm

1 in 10 twist at 4000 fps = 288,000 rpm

Lisa
 
JEC said:
Thanks Jackie...I understand that RPM increases with velocity, but wouldn't the number of rotations the bullet makes over a given distance be the same regardless of velocity...it would just get there in less time?
For example:
In a 1-12 twist, a bullet exiting the barrel into a vacum makes one rotation every foot it moves so it would turn 100 times in 100 ft. Launched at 1fps, it would turn 100 times in the 100 seconds it takes to get to the target. If launched at 2fps it would still turn 100 times but would reach the target in 50 seconds. Since the bullet makes the same number of rotations, regardless of velocity, gyroscopic stability should remain constant.
Now, if we put that same bullet in "air", it is affected by friction for 100 feet, regardless of whether it it is travelling at 1fps or 2fps. Unless increased velocity decreases friction, rotational decelleration should be relative?
Come on engineers...what's happening?
Click to expand...

Fuh'geddabout the travel for a minnit, it IS spinning faster. :) Yes it will have the same rate of travel/rotation but this doesn't negate the fact that it's spinning faster. What you've isolated is why the effect is limited to those combination's which are just on the ragged edge of stability. To illustrate in reverse..... I've got a 6PPC which has a twist of about 14.5 and I used to shoot a particular 71gr varmint bullet. I went out to shoot when it was 45degrees and the bullets keyholed. It's repeatable, it'll stabilize at 70*F but not at 40*F at approximately 1000' above MSL. When the air gets fat it tips over. The rotation is sufficient to stabilize at higher altitudes too.......And of course its stability gets better as it slows down, stability increases with range.

Evidently it's faster ENOUGH to overcome the difference in air resistance. (IF this is even a factor.) Beyond this I'm completely lost. When The Guru's begin to talk in terms of SG's and overturning moments it means little to me.....

At the risk of changing the tenor of your thread I'm going to further ask this question...... "Does velocity or air resistance or deceleration even play a large role in this???" The reason I ask is, I've done some low velocity experiments trying to mimic downrange effects and they're weirded by twist rate........ In other words I'd need about a 1 in 5 twist to get the results I want at the lowered muzzle velocity. The one person that I KNOW has the answer is Henry Childs who no longer posts here. My gut tells me that stability is mainly rotation related with little value given to drag and its effects.

I'm sure that Brian Litz can isolate it also, or Randy Robinette.....





As per your last statement, "rotational decelleration should be relative?" I believe the answer to be "no." A bullet loses very little rotational velocity VS forward velocity in flight. They're not rational.



al
 
JEC:

The easiest way to see what effect velocity has on bullet stabilization (the S.G. number) is to go to the JBM site and use the Drag/Twist table.

Input your parameters and get a baseline S.G. number. Then, using the same parameters, simply alter the velocity and see what S.G. you get. After doing this a few times you'll realize that velocity (within the real world limits of the case/bullet weight combo you're working with) has very, very little influence on S.G./bullet stability.

An S.G. number of 1.5 is considered by many as the perfect area to be in for maximum precision. It's no accident that the 6mm/.825 jacket/1:14 twist (think 6PPC) setup and the .30/.925&1.00 jacket/1:17&1:18 setup (think 30BR and it's variations) operate in that range. ;)

In my own experience with the .30's in BR competition, I've used combos with SG's from over 2.0 to barely 1.4 with good results....with the 1.5-1.7 combos being the best performing. I did shoot an entire seaon with a combo giving a S.G. of around 1.35.....due to a 'slow' 1:17 twist and the 1.080 jacket lengths I was using at the time. With it, I won all four IBS 100-200 HBR Grand Aggs at the old Mason City, Iowa range and set a new 100 yd. range record at 100 yds. with a 250-20X.

And while it worked..I wouldn't purposely travel that road again. :eek:

The 2.0+SG setups (1:15's/1.00 jackets) shot well but they definitely weren't as crisp as the stuff in the 1.5-1.8 range. One thing I noticed with the 2.0+ SG combos was the tendancy to never quite be able to get all the verticle out of the groups despite powder charge, powders, neck tension, primers, seating depth...in sort, anything I could change with with normal tuning methods. This was very apparent at 200 yds. in good conditions when testing.

I think what I was really seeing...and I'm somewhat hesitant to post this :rolleyes:...was in fact the result of an overspun bullets tendancy to displace along a comma-shaped arc 'on target'.

R.G. is pretty savvy on this...hopefully we'll hear his thoughts on it.
 
Bullet stabilizing

The main factor regarding bullet stability after the twist rate is the length of the bullet. Weight of bullet has nothing to do with it and velocity much less than one could assume. As a basic rule of the thumb as Jackie said if a given bullet will not stabilize from a given barrel incresing velocity is not going to change that. There are exeptions from that and incresing velocity can help a marginally stabele bullet. One has to have in mind that in most cases from a given rifle the window of incresing velosity from a usable load and to MAX load is relatively narrow. Perhaps 200-300 fps.

On the other hand changing from FB bullet to Boat Tail can result in the bullet not beeing stabilized cuse of Boat tails beeing longer all else even. I have a 25-08 Imprroved the has 1:14 twist it shoots 100gr Sierra Flat base very accurately. Loaded with 100gr Sierra Boat Tail it is all over the paper. The boat tail bullet beeing about 3mm longer and 1:14 very marginal for .25 cal 100 gr bullet. Incresing velocity was not an option as the 100 gr was at about 3350 fps.
 
You asked for an engineer to weigh in, so OK, I'll give it a go. Let's start by thinking about what we're trying to do by spinning the bullet: what's the instability we're trying to fix?

Let me draw an analogy with another hobby of mine, archery. I make my own arrows, and like most fletchers, I put the feathers on the back end of the shaft, not the front. Why? Well, with the feathers on the back, if the arrow isn't pointed exactly downrange, the aerodynamic forces on the feathers act to straighten things out --- the back end gets pulled in line. The arrow is "aerodynamically stable." If I were to put the feathers on the front (or more realistically, if I put a big broadhead up there), then if the front got a bit out of line during flight, the aerodynamic forces would make things worse: the front end would get pushed out even more. Unfortunately, the bullets we use act like this bass-ackward arrow: they're aerodynamically unstable. Unless we do something, they'll tumble in flight.

So how does the spin help? Here think about a kid's toy top. If it's not spinning at all, or just spinning very slowly, and you try to balance it on its point, it will fall over. But if you spin it fast enough, it stays up quite happily. It turns out that the effectiveness of the spin in counteracting the tendency to fall over increases with the square of the RPM's. Get the RPM's up above a certain value (some number that depends on the top's shape and weight), and the top becomes stable. Likewise with our bullets: spin them fast enough, and the gyroscopic effect overcomes the underlying aerodynamic instability.

OK, so what's the velocity have to do with anything? Start with the aerodynamic instability: aerodynamic forces typically grow more-or-less like the square of the velocity, and the bullet's tendency to tumble grows accordingly.

But what about the stabilizing influence of the spin? As noted above, the spin's effectiveness goes up like the RPM's squared. And as pointed out elsewhere in this thread, the bullet RPM's as it leaves the barrel are directly proportional to the muzzle velocity (for a given barrel twist). So, strength of the gyroscopic effect goes up with the muzzle velocity squared.

It looks like we have a tie: increase the velocity by 10% and the tendency to tumble goes up by 21% (1.10 x 1.10 = 1.21). But at the same time the RPM's go up by 10% and the effectiveness of the spin goes up by 21%. So if we were just barely stable at the original velocity, we're still barely stable at the higher velocity. This is why the stability ultimately isn't very dependent on velocity. Things pretty much depend just on the bullet's characteristics (size, shape, density, distribution of mass) and the barrel twist. [As alinwa's example shows, air density can count too, because the aerodynamics forces are proportional to density.]

So how could increasing the velocity help at all? Well, the aerodynamic forces don't increase _exactly_ like the square of the velocity. At supersonic speeds, things may grow a bit more slowly than that. When the velocity increases by 10%, the tumbling torque might go up by only, say, 17% instead of 21%. But the RPM's really do go up by 10% and the effectiveness of that spin by 21%. So if things were barely stable at the original velocity, things might not be quite so touchy at the higher speed. Whether this is observable in practice, I don't know from personal experience, but I've heard the claim more than once. And I've seen measured aerodynamic data (coefficient of overturning moment) that suggest that it should happen, at least for that particular bullet.

John Paul
 
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