TargetTalk

What is the point that a bullet reach's its terminal velocity?
Is a bullet still accelerating after leaving the barrel ? For how far?

Has anyone a copy of "HATCHER'S NOTEBOOK, A Standard Reference Book for Shooters, Gunsmiths and Ballisticians" who can look up the answer ?

Whilst playing with some chronographs, some interesting results and kinks have been occuring that have been difficult to explain.

Why, when a Chronograph with a 24" screen spacing (8-10' in front of the muzzle) gives one reading, then when the screens are increased to 48" spacing (front screen moved 2' closer to the Muzzle (limited by cable length)) did the average velocity increase by 2%?

Tested and measured over 200 rounds with three chronographs, screen distances re-checked, lots of headaches and much coffee.

Also interesting was putting three chronographs inline - result three different reading ???

What a bag of worms the ISSF has opened up!!!!

David M wrote:Why, when a Chronograph with a 24" screen spacing (8-10' in front of the muzzle) gives one reading, then when the screens are increased to 48" spacing (front screen moved 2' closer to the Muzzle (limited by cable length)) did the average velocity increase by 2%?

If you are getting a higher average velocity by measuring closer to the muzzle then this would indicate that the bullet stops accelerating as soon as it leaves the barrel.

This makes sense to me as the only thing making the bullet accelarate is the expanding gas in the barrel. Once the bullet has left the barrel, or very soon thereafter, the additional pressure from that expanding gas will drop to zero and there will be nothing to make the bullet accelerate. (this is just my logic speaking, I may be totally wrong)

David Levene has it right. Think of it this way: The chronograph works by measuring the time it takes for the bullet to traverse a specific distance, i.e. the screen spacing, and then calculates an average speed over that distance using the formula SPEED = DISTANCE/TIME. By the nature of this calculation, the calculated speed is that at the midpoint between the screens. When the screens are at 8 and 10 feet from the muzzle, you are calculating a speed at 9 feet ((10+8)/2) from the muzzle. When you move the front screen to a point 6 feet from the muzzle, the cronograph calculates a speed at 8 feet ((10+6)/2) from the muzzle. Once the bullet leaves the muzzle, the only thing it can do is slow down, since the only force acting in the direction of travel is drag. Therefor, the bullet is faster at 8 feet from the muzzle than it is at 9 feet from the muzzle. Had you obtained the 4 foot spacing by moving the other screen to 12 feet, you would have calculated a speed at 10 feet from the muzzle, and you would have seen a reduction in calculated velocity.[/i]

What is the point that a bullet reach's its terminal velocity?
Is a bullet still accelerating after leaving the barrel ? For how far?

I saw a post on this exact topic somewhere on the 'net a day or so ago and can't find it now!

In fact the bullet still accelerates for a distance past the end of the muzzle (verified by high speed cameras at a ballistics lab...wish I could find it). As the bullet leaves the barrel there is still gas pushing on it, although obviously that dissipates quickly.

My recollection was that distance was a multiple of caliber, so 4 feet from the muzzle the velocity should already be in decline.

There are many variables to consider, but basically, the bullet will only accelerate if the pushing force is greater than the opposing force. If the powder charge is low, the balance point can actually be within the barrel. IOW, the bullet can reach its maximum and start decelerating before reaching the muzzle. Worse case, it stops within the bore. Under normal circumstances there is a sufficient charge, and the pushing force falls away within a very short distance past the muzzle.

Take Care,
Ed Hall
http://www.airforceshooting.org/
http://www.starreloaders.com/edhall/

Some salient points to consider:

1) the bullet stops accelerating horizontally "almost immediately" (within the caliber radius of the muzzle; i.e. millimeters not feet) upon clearingt he muzzle and decelerates horizontally from then on

2) the bullet begins accelerating vertically (down) upon exiting the muzzle

3) all chronographs have measurement error; e.g. the same exact chronograph will give different readings for the same exact bullet and velocity. For the highest quality chronographs this can be oom 5%

4) the measurement error between chronographs can be quite large, so that even if you overlapped the instruments (first screen of A-first screen of B as close together as possible, then some distance, then second screen of A -second screen of B as close together as possible) the recorded velocities for the same projectile would differ. For the highest quality chronographs (of teh same brand) this difference can be oom 10%; between brands 15%

5) A chronograph needs to be some distance from the muzzle (10-15 feet) to give a reliable reading inteh first place, to avoid errors induced by picking up shock effects and bits of debris flying across the screens

O.K., so under the best possible conditions (overcast sky, best quality chronograph, placed 13 feet from muzzle) you will need to shoot around 20 shots of hte same exact load and take the average and standard deviation of the shots to get an ESTIMATE of the "true" velocity. Then, you will need to make sure the average of *your* load is at least 2 standard deviations *higher* than any minimum rating factor you are trying to achieve. Even at that, you run a non zero chance (due to dissimilarities between chronographs) that you will be called in non-compliance.

Steve Swartz

(it's a little more complicated than that, and others are certainly welcome to quibble ont eh details, but I have captrured the gist of it)

Steve, Most of if not all the problems you are having with cronographs are eliminated if you use an infrared cronograph.
My Ced Millennium is very conistant, day to day, month to month, and of course you can use it indoors for even better control of conditions.
Cheers
Dave C

Sorry if you thought I was "having problems" with chronographs. My comments applied to *all* chronographs. The limitations of chronographs are not a problem per se.

I'm somewhat intrigued by your comment about the "consistency" of your chronograph. How would you measure a concept like "consistency," given that the thing you are measuring (velocity of a projectile) is in itself not consistent at all to start with?

Steve Swartz

(Note: precision is the ability of an instrument to measure the same exact parameter over and over again and give the same measurement; accuracy is the ability of the instrument to measure a parameter any individual time and return a value very close to the "true" value of the parameter. Both precision and accuracy are good things. How do you think the precision and accuracy of a chronograph that gives consistent readings of different values stacks up?)

Steve. As you stated, Consistency: The same results over and over again using the same equipment and ammo…
My Chronograph has given me the same readings over and over again, Standard deviation of 2 over 10 shots. Over many months, and in different locations. Walther LX300xt, JSB lights. Walther Dominator, JSB Heavy’s, Anschutz 54 action, Eley eps. All at 10 feet. (I measure to the front element every time, elements are 2 feet apart). The .22 has a greater deviation but the results are consistent, as long as the temperature is the same. Most of the readings for the Air rifles are taken at my own indoor range. I think because I am using a constant light source, i.e. the IR LED’s, most of the random elements of available light are eliminated. No real technical research to back up my statement, but I’m very happy with the results.
I have used other Chronographs and as you said, weather plays a major part in the results.

I know you are going to consider this "quibbling" but here goes anyways.

I never said- because, quite frankly, it isn't true- that consistency is " . . . the same results over and over again using the same equipment and ammo . . . "

What you seem to be referring to is a common error in measurement based on the concept of "floating calibration."

Why should you assume that the "same ammo and equipment" is in itself some kind of universal constant? Kind of like "calibrating" your speedometer by following a buddy who also has an uncalibrated speedometer. All you are doing is calibrating your error to his error.

Anyhow, yeah this is quibbling. Sorry; I'm having an Engineering Flashback.

Steve Swartz

What "can of worms" are you refering to?

What I haven't seen is any discussion of how accurate, how precise, and/or how consistent (3 different things) your chronograph and velocity measurements need to be. If a difference of 10fps doesn't move the position of the shot on the target at least 1/10th of a pellet/bullet diameter, does it matter to your score? If not, will you pay 10 times as much for your 'graph to measure velocity with an accuracy of 1fps and a precision of.01fps? I sure won't!! It may be fun but its operationally meaningless. Its hard, but valuable to determine what values really affect your score. Oh yes:
ACCURACY measures how close the number is to the "true real world" value
PRECSION measures what the smallest discriminable difference in measured values is - 0.2 fps is 10 times more precise than 2.0 fps, but may not be any more accurate or consistent.
CONSISTENCY is the repeatability of a measurement - given a reading, how close to the same value does the system record over a series of readings>
[sorry, I had a metrology/experimental design flashback)
Richard Newman

Richard:

Thanks for the much better definions of precision, accuracy, and consistency (what is commonly called reliability of measurement in other disciplines).

I think the issue here is not necessarily holes in the paper per se but meeting velocity requirements for disciplines when 1) you want to shoot the lowest recoiling ammo possible, but 2) the rules stipulate a minimum allowable velocity.

If *your* chronograph and the *match directors/judges* chronographs err in different directions, you could easily find yourself disqualified over a "measurement error."

The difference of 10 fps could invalidate your score.

Using commercial ammo is not considered a defense . . . I think this is the bag of worms referred to.

Steve Swartz

"the bag of worms to which you referred" :-)

A dangling preposition is an error up with which I will not put.

The "bag of worms" I opened was that the other weekend at a Service Pistol Competition (similar to PPC). I DSQ four competitors because the ammo did not make the required "Power Factor".

The factor of 120,000 is defined as bullet wt (grains) times Velocity (fps).
One competitor was rechedcked with a second chrono and still failed.

Later, three chrono's were put in line and although each chrono was consistant within limits with itself, shot to shot, the difference between chrono's was vast.

I then bought a new CED chrono and added it to the test and got different reading to the other chrono's as well as over different screen spacings.

More testing , including with screen at 6' and 8', as well as using a airpistol (for least variable velocity) and any other thing I can think of ie. bright,dull,infered, and amber screens.

So the bag of worms is basically messy and all over the floor.

The ISSF rule of a cerfified chronograph is looking pretty sick right now.

Could be interesting results over the next couple of months.

As there is no procedure for field calibrating the chronographs, and as there no standards for calibrating the chronographs between manufacturers, the situation you describe is entirely to be expected.

Just out of curiosity- what exact levels of disagreement were you finding? How many shots did you collect from each competitor/load to test for compliance?

Steve Swartz

David M wrote:The ISSF rule of a cerfified chronograph is looking pretty sick right now.

Funnily enough there used to be a thread over on the ISSF News forum, started by Spencer, about this very subject. He (and I later joined in) wanted to know how the certified chronograph had to be calibrated.

As it happens, that thread seems to have disappeared (while older and newer ones are still there).

Far be it from me to start a conspiracy theory but......... ;-)

David Levene wrote:Funnily enough there used to be a thread over on the ISSF News forum, started by Spencer, about this very subject. He (and I later joined in) wanted to know how the certified chronograph had to be calibrated.

As it happens, that thread seems to have disappeared (while older and newer ones are still there).

Far be it from me to start a conspiracy theory but......... ;-)

Can you not re-post the same question ? I've never used their forum so I don't know how it works.

Rob.

RobStubbs wrote:Can you not re-post the same question ? I've never used their forum so I don't know how it works.

Head and brick wall Rob. They ignore direct email requests for rule clarification so a question on the ISSF News forum has little chance of success.

It's technically an academic question anyway as the procedure they issued for testing ammunition was only valid for 2005 World Cups. I have emailed them asking for confirmation that it is extended for this year (as I presume) but guess what, they haven't answered that email either.