Shooters Forum banner
1 - 20 of 23 Posts

·
Registered
Joined
·
774 Posts
Discussion Starter · #1 ·
Did they calculate it? If so, couldn't have been very accurate.
 

·
The Hog Whisperer (Administrator)
Joined
·
37,402 Posts
Pendulum "chronographs."
 

·
Registered
Joined
·
774 Posts
Discussion Starter · #3 ·
Sorry, posted that twice. Please delete. Thanks
 

·
Registered
Joined
·
774 Posts
Discussion Starter · #4 · (Edited)
How did pendulum chronos work? Physics, and some great mathmeticians.
 

·
Registered
Joined
·
172 Posts
ONE method sometimes used, was to fire several projectiles, level with a precisely-leveled floor through a path of multiple thin sheets of paper over a set distance. The drop of the projectile would be measured and averaged. Using kinematic equations, they could back-calculate the initial (muzzle) velocity in parabolic flight.
We did this in high school, using a Daisy BB gun and onion-skinned paper. We got within 6% (low) of the results the owner of the only (and at the time VERY expensive) chronograph in the county obtained. He was quite amazed, but also remarked that he did not do well in physics.
 

·
The Hog Whisperer (Administrator)
Joined
·
37,402 Posts
Yeah but then the math might make your head hurt ;)
 

·
Registered
Joined
·
9,698 Posts
Until about 1950’s-1960’s...we just took their word. Maybe 1/10th of 1% actually checked, and they mostly relied on drop, BC estimates, and math.

Besides calculations, the earliest “home made” direct results I can remember seeing (drawings of) relied on:

1. Gravity and a dropping weight that was mechanically started and stopped (or at least marked if not stopped). Still some calculations, error from the mechanical lag-time, but given gravity as a constant, it kind of works to give the time of flight.

2. (electric) two spinning identical disks, spinning at a known RPM...shoot though both and figure the difference in impact on the graduated disk. How they figured a uniform speed wasn’t explained.

3. Working backyards from a momentum pendulum. Know the weight of the bullet, can get a pretty good estimate (at lease until you deform the plate from speed). Did lead the British into a whole “transition dwell” theory, which is pretty much a dead end today.


Even in 1965-1970, a chronograph was so rare as to make a whole shooting range sit up and take notice. Expensive (way more than a new rifle), wire screens that had to be changed after each shot, non-digital read out.

Early-middle 1970’s worked out “sky screens”...even more expensive units, but at least you didn’t have to wait for a cease fire to go out and change printed wire-screens after one shot.

Eventually it got cheap and simple….bought an air gun Chrongrph that actually works for all of $26....and now we’re kind of slaves to the numbers.

Seriously concerned with 2765fps rather than the 2810fps we expected….energy differences that are basically worth a .22 CB cap….1 ¼ % variation rather than 1 5/8%...all sorts of minor BS
 

·
Registered
Joined
·
236 Posts
Yes I have seen some information on the spinning disks. The disks are at a known distance apart and are moving at a known RPM. I envision them on a common shaft. The bullet offset from the first target to the second target is some degree or fraction of a RPM. As RPMs has a time element to it and the distance is known/fixed the speed of the bullet in feet/sec. (or any unit of speed) could be calculated. I have seen pictures of it. It was so long ago I don't recall the details.
 

·
Registered
Joined
·
774 Posts
Discussion Starter · #11 ·
I know sometimes companies would advertise a velocity for a new cartridge, and it would be way off what they advertised. Examples, .45 Colt 255 gr. lead bullet at 810 fps. I've read where shooters have loaded ammo with 35 gr. black and 255 gr. lead, and shot out of 7 1/2 " barreled Colt clones or maybe originals and got something like 940 fps. Another was the .220 Swift. 48 gr. bullet at 4140 fps. I'm pretty sure it didn't do that.
 

·
The Shadow (Moderator)
Joined
·
8,884 Posts
I know sometimes companies would advertise a velocity for a new cartridge, and it would be way off what they advertised.
That is still true of anyone who isn't Sierra, and states a single G1 on most rifle bullets🙄😆😆

Cheers
 

·
Registered
Joined
·
1,769 Posts
Did the old Powley computer give FPS estimates? I can't remember.
 

·
Super Moderator
Joined
·
13,747 Posts
Yep, spark gap. It's an old method of timing. There was an instrument demonstrated to my physics 101 class in college that used a waxed paper tape pulled through a 60 Hz spark gap to demonstrate measuring gravitational acceleration by the increasing distance between spark holes made in the paper as the tape was pulled through the gap. We had lots of antique lab gear, I recall.

This old Encyclopedia Britannica entry from 1911 lists 22 different chronograph methods. Back then, chronography was an entire field of study and research unto itself. Obviously, the development of electronics added more methods. The earliest one I was familiar with was published as a do-it-yourself project in Popular Electronics in the late 1960s. It was a true chronograph in that it was a form of electronic stopwatch using early resistor-transistor logic ICs. It counted cycles from a crystal oscillator as the bullet traveled between screens that, IIRC, were made by gluing aluminum foil to opposite sides of a piece of cardboard so the bullet would cause a momentary short circuit between them. The display was four or five digits each comprised of ten lights corresponding to 0 through 9. You had to divide the distance between the screens by the time displayed to get feet per second. It did not do that math for you, as modern chronographs do. We are spoiled, now.
 

·
Registered
Joined
·
1,947 Posts
Yep, spark gap. It's an old method of timing. There was an instrument demonstrated to my physics 101 class in college that used a waxed paper tape pulled through a 60 Hz spark gap to demonstrate measuring gravitational acceleration by the increasing distance between spark holes made in the paper as the tape was pulled through the gap. We had lots of antique lab gear, I recall.
 

·
Super Moderator
Joined
·
13,747 Posts
Hah! Yes! That's the beastie! Many years later, I advised a small group of engineering students doing a project for a wallpaper company on how to make a precisely directed spark perforate wallpaper stock without burning it. So the knowledge came in handy (as did some of my intervening year home studies of Tesla coils).
 
1 - 20 of 23 Posts
Top