How did they measure FPS?

[rauchman]

Greetings,

In lurking on this and other boards, I'm surprised to come across info where ammo companies had FPS data on really old cartridges. How, for example, did they measure the FPS of a .45 Long Colt in the 1880's? With radar and lasers today, it is easily understandable how to calculate this type of data. However, without lasers and radiosignals, how did they figure this out way back when?

Ken

 

[Mal H]

The science of ballistics progressed rapidly in the 1800's. To find the velocity of a projectile, they used a device called a "ballistic pendulum". Briefly, a bullet was fired into a heavy suspended box that would capture the bullet. The bullet's forward momentum was transferred to the box and it would swing a certain distance from perpendicular. Knowing the weight of the bullet, the weight of the box and the distance it swung, they could determine the velocity of the bullet.

Don't forget calculus and Newton's laws were well established long before the 1800's.

 

[Colt46]

That seems too easy an answer

I wanted something more like wizardry!
I hope the pendulum device was more accurate than modern means of measuring velocity. Factory numbers rarely seem to be obtainable with modern firearms.

 

[Jim Watson]

Late 19th early 20th century ballisticians had the Boulenge electromagnetic chronograph. See my quickie description at:
http://www.thehighroad.org/showthread.php?s=&threadid=99721&highlight=Boulenge

There were other designs because there was a lot of interest in the power of guns then as well as now.

The real reason factory advertised velocity is not often seen in mass production guns is because factory velocity is taken from a P&V (Pressure and Velocity) barrel made to minimum bore and chamber dimensions, which tend to increase pressure and velocity, and also probably longer and with smoother internal finish than your sporter.

 

[mondocomputerman]

We measured it in my Physics classroom in college (1998) The teacher actually brought in a .22 revolver and fired it into a block of wood suspended from the ceiling. we measured the weight of the block of wood and already knew the weight of the bullet. We measured how far the block of wood raised from the floor and did the math. Never would have thought he would have fired in class...

 

[Preacherman]

Actually, they hired some very fast runners with early-model stopwatches to follow the bullet...

 

[JohnKSa]

Ballistic Pendulums are not anywhere near as accurate as modern chronographs.

There is energy lost in deforming both the bullet and the pendulum, and that causes a loss in momentum. A ballistic pendulum will always read low for that reason, and the amount it reads low will vary based on a lot of things.

It's probably pretty good for relative measurements of similar bullets fired at roughly similar velocities.

 

[Rockstar]

Hey, we got guys now who think that bullet speed is measured with radar and radio signals!

 

[TrapperReady]

I thought that the low readings from the ballistic pendulum method were always "corrected" by the guys in marketing.

Also, the manufacturers could just see what the published numbers for a similar round from a competitor was listed at... and then add a couple hundred feet per second. Not like that would ever happen. Right????

 

[P95Carry]

What they really did in old days ... with the trusty front-stuffers and Minnie bullets ... was ....... they put a guy down range at 1000 yards ... and he timed the sound of a bullet impact against the sound of the shot fired. He then calculated the result, adjusting for the speed of sound (approx 1100 fps) to reach an approximation for actual bullet terminal velocity. Whether he adjusted also for extant barometric conditions and height above sea level I am not sure.

Maybe if the time lag between impact sound and gun shot sound was minimal ... he'd reckon the Minnie was cruising at about 1,000 fps or just over! He then did more math and backtracked so as to deduce the probable muzzle velocity able to achieve such.

(Yeah - and if you believe that - you'll believe anything!!)

Gimme my Chrony any day!!

 

[TimRB]

"We measured it in my Physics classroom in college (1998) The teacher actually brought in a .22 revolver and fired it into a block of wood suspended from the ceiling."

While I think that would be a splendid demonstration and exercise if done properly, from your description it's not clear what he actually did. Did he at least set up some sand bags or some other bullet trap? Was everyone wearing ear and eye protection?

Tim

 

[unspellable]

Ballistic pendulum

<< Ballistic Pendulums are not anywhere near as accurate as modern chronographs.

There is energy lost in deforming both the bullet and the pendulum, and that causes a loss in momentum. A ballistic pendulum will always read low for that reason, and the amount it reads low will vary based on a lot of things. >>

Not so. Energy transfer or expenditure has nothing to do with it. There is a law of physics called conservation of momentum.

Before impact the bullet has a certain momentum, its velocity times its mass. The pendulum is stationary and has zero mementum. After impact the total momentum is the same as before impact. Multiply the velocity of the pendulum by the combined mass of the pendulum and bullet and divide the result by the mass of the bullet to obtain velocity.

I did this one in Physics class also. There are some losses, but they are not due to the bullet energy lost in deforming the bullet or the trap. (The energy to do this results from the bullet loosing velocity.) There will be an error due to friction because one usually has the pendulum push a small sliding block on a scale to measure the distance the pendulum has swung. If done properly its quite accurate. There is no great difficulty in achieving an accuracy of plus or miunus 2%. One could do better with a fancy setup designed for the purpose.

 

[JohnKSa]

The energy used to deform the bullet and the pendulum as well as the heat generated from the collision has to come from somewhere.

The deformation and the heat generation is a result of the conversion of kinetic energy. The mass of the system doesn't change in that process--the only other variable in kinetic energy is velocity. If some of the velocity is being used for deformation and to generate heat then it has to come out of the mass velocity product used to calculate the momentum transfer.

 

[Mal H]

"Ballistic Pendulums are not anywhere near as accurate as modern chronographs."

Come now, John! Usually a statement like that is followed by the term - Duh! You might want to look up the word "nit" and think about how you are picking them.

When it's the only thing you've got, you use it. Along with the Le Boulenge chronograph, the BP was about the only consistent methods available in the 1880's.

It's true that some of innaccuracy was due to the instantaneous interactions of the bullet and the pendulum itself. But, those were fairly small compared to the inaccuracies introduced by the air friction, pendulum rod axle friction, difficulty in measuring distance traveled, inaccuracy of weight measurements, etc.

I seriously doubt that the heat developed when the bullet hit the pendulum would change the reading by 1 fps, and only if all the other factors were 100% accurate.

When a bullet is deformed, the energy that goes into that deformation is released in heat. The mass of the total system is the most important factor and that doesn't change no matter how much the bullet is deformed, assuming all of the bullet remains within the system.

If you want to get down to the physics of it all, you could apply a Lorentzian transform to the heat function to see how much mass was lost. Without knowing the mass figures or the heat produced I can't do that, but, I can pretty much guarantee it will be in the microgram amount.

 

[Jonathan]

175 grains at 2600fps
(0.0113398 kg at 792.48 m/s)
E = 1/2 m v^2 = 3560.8364 J



deformable wood block, 1 kg, 0 m/s
E = 0 (ignore potential)


impact with conservation of momentum:
1.0113398 kg at 8.8858 m/s
E = 39.9264 J



Why worry about exchanging mass for energy? There's plenty of energy to go around.

Momentum works.

 

[JohnKSa]

My reference to the accuracy of the ballistic pendulum was in response to an earlier comment on the thread about the accuracy of the pendulum compared to modern devices.

And, surprisingly (to me anyway) the deformation of the projectile and the pendulum are irrelevant as long as there's no splatter. Sound and heat do matter, but are small contributors, I suppose.

 

[unspellable]

Pendulum

First, the pendulum bob is in the form of a bullet trap. When the bullet is trapped momentum is (exactly) conserved. It is momentum we are measuring. Thus the method can be quite accurate.

If you do the math, energy is lost. But not momentum. It is this energy loss that goes into deforming the bullet, noise, heat, etc. I think the confusion arises here because of failure to distinguish between energy and momentum.

I can demonstrate the math here if any body really wants to be bored.

 

[Jim Watson]

Yes, but if the setup is not carefully made and operated friction can transfer momentum to the planet where it is not readily measurable. This would lead to a low reading for bullet velocity, not exaggerated, though.

 

[Archie]

Another system...

was a device with a ribbon.

The bullet tripped a falling weight that pulled a ribbon, then the bullet tripped a second device that stopped or cut the ribbon. By measuring the amount of ribbon pulled, one could calculate the time between start and stop and from that, the velocity.

And, as mentioned by Trapper Ready, the guys in marketing and ad copy writing made the final 'adjustments'.

I personally, am delighted about having a Chrony.

 

[HankB]

Well into the 20th century, the military measured velocity at 78 feet. How did they do this, and why such an odd number?

Well, they took a metal rod some 150 feet long. They put a disc of cardboard at each end, and then they set the rod to spinning around its axis at a known rate.

When they fired a rifle bullet through the system, parallel to the rod, the bullet sequentially pierced the first disc and then the second. But since the whole system was spinning, there was an angular offset between the holes in the first and second discs. Knowing the rate at which the system was spinning, they were able to use this angular offset to compute a time of flight. Knowing the time of flight and the distance, it was a simple matter to compute the average velocity over the 150 foot path between the two discs. They assumed this average velocity was the velocity at the midpoint of the system, some 75 ft past the first disc. (This is not exactly correct as drag forces vary nonlinearly with velocity, but in this case it literally WAS "close enough for government work." )

The first disc was 3 ft from the muzzle, so they added this in and, presto, velocity at 78 feet.

 

[MrAcheson]

"The energy used to deform the bullet and the pendulum as well as the heat generated from the collision has to come from somewhere."

Yes they come from the energy of the bullet, but mechanical energy is not conserved in a collision like this anyway. The momentum the bullet the instant before it impacts and the momentum of the bullet and pendulum the instant after the bullet impacts must be the same (or so close as to make no difference). Its the Law of Conservation of Momentum and it must be this way.

The innaccuracies come from friction, air resistance, and potentially issues with unaccounted mass in the system. Deformation of the bullet can't have anything to do with it.

 

[Monkeyleg]

I have no idea how measurements were done years ago, or even now.

My suggestion would be to take that "Can You Hear Me Now?" actor from the cell phone commercials, put him 1,000 yards downrange, and shoot him. Record the first utterance of "Can you hear me..." and the last "Ohhhh...", and then measure the time lapse.