Chronograph Accuracy

[chkunz]

Accuracy reloading depends on many things including our ability to measure and weigh, etc. We have the ability to calibrate our micrometers, calipers, and scales. Most of us do not have the ability to measure chamber pressure.

Muzzle velocity is an important parameter and we have chronographs but we do not have the ability to calibrate our chronographs. To make maters worse chronographs are very sensitive to how we set them up and use them and we can get significant erroneous results from improper set up.

Bryan Litz made the best of the bad situation in his book on Applied Ballistics for Long Range Shooting but he did not have the capability of calibrating his chronographs.

My objective of this post is simply to make folks aware of the uncertainty of our ability to measure muzzle velocity. We depend on an instrument that we do not have the ability to calibrate. That is my point , no more no less.

 

[d.id]

Mine was a misleading son of a b^%$#. Got sick of it's lies.
Switched to...... gravity.......
Drop correction required from 0 to Distance in noted environment x B.C. = True velocity.
Regardless of level, lighting, battery life, etc etc.
Still for the life of me can not conjure a single reason to own one again.
Gravity is faster, cheaper, easier, takes less time and ammo and is allot more accurate.

 

[jcampbellsmith]

Mine was a misleading son of a b^%$#. Got sick of it's lies.
Switched to...... gravity.......
Drop correction required from 0 to Distance in noted environment x B.C. = True velocity.
Regardless of level, lighting, battery life, etc etc.
Still for the life of me can not conjure a single reason to own one again.
Gravity is faster, cheaper, easier, takes less time and ammo and is allot more accurate.

NB: Gravity is not constant and is not always in the same direction. Gravity meters require calibration too. This is a major concern to folk launching ICBMs.

Regards JCS

 

[dmoran]

Currently I own 5 chronographs (have owned more). When setting all 5 inline with setup detail they typically will read with in 1% of each other. Setup is the key -IMO

What I use to calibrate my setups is a 17-HMR with factory Hornady 2550-fps ammo. With my 17-HMR rifle and that ammo will produce ES of 15-fps or less then the 2550-fps advertised. Been using it for calibration since 2006, before that I used a proven Lot of Elloy ammo in a 22-rimfire, which worked well also.

The biggest aspect I want, desire, and demand from any chronograph setup is for them to simulate with the actual POI's on target. Meaning the extremes read as seen on the targets, with the faster shots POI printing higher then the slower, and vise-verse.

For optical sensor type chronographs I strongly advise to setup with infrared lighting and/or false lighting, which I find greatly improves accuracy and repeatability.

My own personal favorite setup is with two CED M1 units, both with IR, setup in tandem like this: 1__2____2__1
(unit-1 at 8' spacing and unit-2 at 4' spacing with in unit-1). Setup to shade out the sun entirely, relying on the IR for lighting only.

In closing and in my own opinion, if we want "laboratory type results", we need to give our setups added attention to do so. The quickly setting them up in the ambient and not taking in account for lighting, levelness, spacing, sun angles, etc.. etc., is the common mistake to setup I see get played out.

My 2-cents
Donovan

 

[dreever]

For me it's just a tool to work up loads. Once I get single digit E.S. and S.D. numbers, and the velocity appears about right I shoot it on paper at my different yardages. If it's shooting little groups as far as I'm concerned I'm done.

Danny

 

[Flouncer]

NB: Gravity is not constant and is not always in the same direction. Gravity meters require calibration too. This is a major concern to folk launching ICBMs.

If you shot your development loads at the same place, the same geographic location, like at the same shooting range, how would gravity difference be a factor ?

For a bullet at said location, shot horizontally in the same vector, same distance say 100 yards, how would gravity vary enough to have any effect ?

 

[CatShooter]

Mine was a misleading son of a b^%$#. Got sick of it's lies.
Switched to...... gravity.......
Drop correction required from 0 to Distance in noted environment x B.C. = True velocity.
Regardless of level, lighting, battery life, etc etc.
Still for the life of me can not conjure a single reason to own one again.
Gravity is faster, cheaper, easier, takes less time and ammo and is allot more accurate.

NB: Gravity is not constant and is not always in the same direction. Gravity meters require calibration too. This is a major concern to folk launching ICBMs.

Regards JCS

ICBMs... really????

I'm not sure how many guys test ICBMs on this site (kinda irrelevant, ya thunk??)...

... but out of curiosity, just exactly how much does gravity vary over 100 yards, 1000 yards. and from Connecticut to Pennsylvania, or South Dakota??

I mean, I live in Connecticut, and if I do drop tests and make a drop chart here, and go to Pennsylvania for Woodchucks, or South Dakota for PDs, how much correction to I make for 300, 500, 700, and 1,000 yards, and will the gun shoot high (less gravity) or low (more gravity)??

 

[JPeelen]

When you go south, gravity will decrease, being smallest at the Equator. Going up also reduces it.

For 7.62 mm NATO at 600 m I computed the effect between German coast and a shooting range in the Alps (roughly 600 miles south, 1000 feet higher) as one tenth of an inch. Forget local effects of gravity. They disappear in the effects of changes in temperature and air pressure.

 

[JPeelen]

Making conclusions regarding changes in drag based on changes in drop depends very much on the group size.

I recommend the following test: modify the ballistic coefficient by say 5 percent (which is a lot) and compute the resulting change in drop for a given distance.

Then compare this figure with the task to shoot groups small enough, required to be able to determine the center of impact more precise than the expected change in drop.

I doubt that changes in drop can in practice be measured precisely enough to come to reliable conclusions on drag variations.

 

[jds holler]

This is great! I tuned in on this thread to learn a little more about chronographs, and learned that there is less gravity at the equator.

Won't affect my life in the least, but I always enjoy gaining little bits of knowledge wherever it is found. Thank you. jd

 

[Syncrowave]

Gravity Map of the U.S.:

There are also magnetic anomalies that can cause problems for mariners – these are commonly noted on nautical charts. Also, the molten core of the planet is in motion, which changes where magnetic north is located. Here in Virginia, compass variation (the difference between magnetic north and true north) is about 11° W and that variation increases by about 2 minutes per year.

 

[jds holler]

Over here in SE Oregon, I frequently hear of guys slaughtering squirrels at 300 yards with their 17 HMR's. It must be because of the low gravity. : jd

 

[dmoran]

Maybe sanctions should start handicapping ranges by gravity.
Where to say a range with more gravity pull receives more handicap, to make up the indifference to the scores and group sizes.
Donovan

 

[m500]

It appears the map is showing mGal. Could you explain this unit of measurement?

 

[dmoran]

It appears the map is showing mGal. Could you explain this unit of measurement?

The gal is named after Galileo Galilei, a physicist who made the first measurements of the Earth's gravity.

""" The gal is a unit of acceleration used extensively in the science of gravimetry. The gal is defined as 1 centimeter per second squared (1 cm/s2). The milligal (mGal) and microgal (µGal) refer respectively to one thousandth and one millionth of a gal.

The gal is a derived unit, defined in terms of the centimeter-gram-second (CGS) base unit of length, the centimeter, and the second, which is the base unit of time in both the CGS as well as the modern SI system. In SI base units, 1 Gal is precisely equal to 0.01 m/s2.

The acceleration due to Earth’s gravity at its surface is 976 to 983 Gal, the variation being due mainly to differences in latitude and elevation. Mountains and masses of lesser density within the Earth's crust typically cause variations in gravitational acceleration of tens to hundreds of milligals (mGal). The gravity gradient (variation with height) above Earth's surface is about 3.1 µGal per centimeter of height (3.1×10−6 s−2), resulting in a maximum difference of about 2 Gal (0.02 m/s2) from the top of Mount Everest to sea level. """

 

[jcampbellsmith]

Continuing on the gravity theme. I was brought up very close to this mountain and have climbed it several times.

http://en.wikipedia.org/wiki/Schiehallion_experiment

Regards

JCS

 

[m500]

Dmoran; Thank you for your explanation of mGal. I remembered hearing that satalites experience about 98% of earth's gravity while in orbit, so I knew an mGal had to be quite small, but had hot heard the term before.

 

[d.id]

WOW!
Gravity map discussion......Somebodies got way to much time.

 

[JPeelen]

I think he simply meant that the case needs longer to fall to the ground than the bullet needs to reach the target.

Gravity is a fascinating subject. But as I wrote before, its variations have no measureable effect on rifle shooting.