About these ads
RSS

Chronograph Accuracy Tips – 15 Practical Tips to Increase Accuracy & Reliability

20 Jul

“No mere gadget, the chronograph is one of the most powerful tools imaginable in load development and problem diagnosis. A bullet’s velocity is one of the major contributors to its behavior, and if you don’t know what its velocity is, you may never understand that behavior. But knowing why the bullet behaves as it does, you can take intelligent corrective action that wasn’t available to you before.”
– Dan Hackett, Precision Shooting Magazine

Chronograph ErrorA chronograph can be a powerful tool … but if you get more than a few “ERROR” readings in a row, you may be tempted to put a bullet right through it.

Most of these tips are intended for chronographs that use a natural or incandescent light source (Ex: Shooting Chrony, Competition Electronics ProChronoCED Millennium 2Oehler 35P, etc) and may not all be applicable to chronographs using infrared light sources or other types of projectile sensors.

  1. You’ll get the best readings with even lighting.  Best case scenario is overcast skies, and worse is direct sunlight on the skyscreens.
  2. Remove diffusers in overcast or shaded conditions.  The diffusers are only necessary in sunny conditions, and by removing them in other conditions you’ll allow more light into the chronograph.
  3. On sunny days place the chronograph in the shadow of a building or opaque wall.  Ensure the sensors have a clear view of the sky, but the chronograph itself is in the shade.  You could alternatively add sun shields to create a shadow (see Advanced Techniques section for more details).  The goal is to eliminate direct sunlight on the chronograph, while still ensuring the sensors have a direct view of the sky.  However, avoid placing the chronograph in the shade of a tree.  The uneven, dappled sunlight that filters through the tree will cause more problems than it will solve.
  4. Avoid low light conditions, such sunrise and sunset.  During those times the extreme low angle of the sun can cause additional issues due to reflections.
  5. Clean skyscreens.  If you’ve used your chronograph more than a dozen times, there is likely a build-up of dust on the sensors.  Use a Q-tip or canned air to clean the lenses, being especially careful not to scratch them.
  6. Ensure chronograph is level
    The formula the chronograph uses to calculate the velocity always assumes the bullet path is perpendicular to the skyscreens (i.e. like Path 1 shown in green).  That means the chronograph assumes the bullet travels exactly 12.0” from the time the bullet was seen passing over point A until the chronograph saw it pass over point B.  If the bullet was shot at an angle, it would have to travel farther (in our example ½” farther) and would therefore take longer to travel between point A & B.  So for Path 2 the chronograph would record a slower time, but since it thinks the bullet just traveled 12” … it would say the bullet was going slower than it really was.  Here is a concrete example:
    Times chronograph recorded for the bullet to travel between points A & B:
    – Time 1 = 0.000333 seconds (Path 1, green line)
    Time 2 = 0.000347 seconds (Path 2, red line – longer because it had to travel slightly farther)
    Angled Chronograph Calculations
    The unlevel bullet path in this example would result in a chronograph reading that was over 4% slower than it was actually travelling.  Although I used an extreme example to illustrate the concept, even slight angles can have dramatic effects in the velocity readings the chronograph produces.
  7. Ensure sensors aren’t tilted.  If using a Shooting Chrony, ensure it is completely unfolded.  This would result in a similar margin of error as the example above, because the real distance the bullet traveled doesn’t match what the chronograph is assuming it did.  But in this example, the reading would actually be around 4% faster than the bullet was really travelling.
    Effects Of Tilted Sensors On Chronograph
  8. Position the chronograph 15-20’ from the muzzle to avoid interference from muzzle blast.  To get true velocity at the muzzle (what most ballistics programs need), you can typically add .75 mph per foot.  For example, if the chronograph was 20’ from the muzzle and displayed a reading of 3,200 fps you would just add on 20*.75, which is 15.  So your true muzzle velocity would be 3,215 fps.
  9. Paint all surfaces flat black to avoid erroneous readings caused by reflections, including all chronograph and tripod surfaces that would be in direct line of sight with the bullet as it passed over the chronograph.
  10. Use a quality tripod for a stable platform.
  11. Avoid shooting over reflective surfaces like snow or some types of sand.  May be able to reduce reflection by laying out a large, dark colored tarp under the chronograph setup.
  12. Replace batteries regularly to ensure stable power.
  13. Understand the flight of your bullet will be lower than your sight picture.  Most scopes are mounted 1.5” higher than the centerline of the barrel.  Always aim much higher than the middle of the shooting area.
    Point of Aim vs Actual Bullet Path
  14. Avoid using near florescent lamps.  Although we can’t detect it with our naked eye, they actually vary in intensity many times a second and chronographs will detect these changes rendering them virtually useless.
  15. To work indoors, chronographs need a lot of even lighting … which is hard to pull off with incandescent lamps.  A white background with even amounts of light over the sensing area works best.  Some chronographs offer optional accessories that help make them more reliable indoors.

Advanced Techniques

Attach flat black sun sheilds made of foam board or cardboard to the guide rods to create a shadow.  I bought 2 pieces of black foam board for $4 at a craft store for mine.  This will help prevent direct sunlight from shining on the chronograph, but still ensure the sensors have a direct view of the sky.  Warning:  In windy conditions, these shields could transform your chronograph into a very expensive kite.

Knowing that chronographs prefer even lighting, you can also help by adding a translucent light diffusing cover just a few inchces above the included diffusers.  This would only be necessary on sunny days.  The plastic diffuser sheets commonly used flourescent lighting works well for this, and can be found in 2’x4′ sheets at local hardware stores.  Note:  You shouldn’t mount or even attach the diffuser sheet directly to the chronograph.  Instead create a frame that holds the diffuser sheet, so you can simply set up the chronograph underneath it.

If you are using a CED Millennium Chronograph, you can purchase upgrade parts to replace the standard visible light sensors with infrared skyscreens.  Infrared sensors eliminate the need for sunlight, and can even record velocities in total darkness … which is exactly what some people opt to do.  If your chronograph has infrared sensors you can actually place it in what is referred to as a “coffin” with shoot-through windows, and virtually eliminate all problems caused by lighting and reflections.  Coffin setups only work with infrared skyscreens, so this setup wouldn’t work for the Shooting Chrony or ProChrono.  Here is a good example of a coffin I found in a forum post:
Chronograph Coffin for infrared skyscreens on CED Millennium and Millennium 2 - Setup Overview
Chronograph Coffin for infrared skyscreens on CED Millennium and Millennium 2 - Shoot-Through Windows

To increase the accuracy of the velocity readings (or at least your confidence in the measured velocity), shoot through 2 chronographs in a “close tandem arrangement” to ensure they register the same velocity.  You should add 2-4 fps to the velocity reading of the chronograph that is further downrange to account for the slight velocity loss that occurs in just a few feet.  You probably shouldn’t see more than 5 fps difference after the adjustment.

Dig Deeper

For more info on chronographs, I suggest reading the Oehler 35P Instruction Manual … it contains a ton of great info written first-hand from the leading expert in this field, Ken Oehler.  Most of the info is applicable to any chronograph, not just the 35P … although the 35P is the best personal chronograph in the business.

About these ads
 
6 Comments

Posted by on July 20, 2012 in Chronograph, Data

 

Tags: , , , , ,

6 responses to “Chronograph Accuracy Tips – 15 Practical Tips to Increase Accuracy & Reliability

  1. Unclenick

    December 5, 2013 at 8:49 am

    All good information and tips. I’ll toss in a few more:

    I set up a rifle on bags at the bench so that the sights stay on target after first inserting an empty chamber indicator (ECI) in the open action and a laser bore sighter in the muzzle. I then go out and adjust the height and tilt of the chronograph until the laser dot is showing up on my palm at the same correct place over each sensor. The ECI’s flag has the words “REMOVE LASER FIRST” printed on it in felt marker to remind me not to shoot out the bore sighter (which a web search shows can burst barrels).

    The CED web site page for the Millenium 2 chronograph has a list of eleven error causing factors about 2/3 of the way down the page. Worth a read. It’s mention of temperature is important as most chronographs use the lowest consumer grade electronic components not rated to operate below 32°F. Batteries also increase in internal resistance as temperature goes down. This renders them less able to provide adequate current, so a weak battery may work fine at room temperature but not down near freezing.

    Because most .22 Long Rifle cartridges don’t make enough gas to continue accelerating their bullets after the first 15 to 20 inches of barrel length and since the last of the acceleration just before that point and the slowing of the bullet after that point are gradual, most .22 LR match grade ammunition will shoot to within 50 fps of the velocity claimed by the manufacturer in most rifle barrel lengths. Taking a .22 rifle and some match ammunition to the range makes for a quick double check that your chronograph’s accuracy is not way off the beam. Otherwise, shooting through two chronographs of different makes in tandem is a good double-check to show that both are working properly under the particular conditions in question. I’ve seen up to 200 fps difference doing this in late afternoon light, proving at least one of the chronographs in question wasn’t up to snuff in that light.

     
    • Cal

      December 5, 2013 at 9:00 am

      Great info … Thanks for sharing!

       
  2. JCS

    February 1, 2014 at 1:43 pm

    Try one of these: http://www.magnetospeed.com/

     
    • Cal

      February 1, 2014 at 4:27 pm

      Paul from AccurateShooter.com (aka 6mmBR.com) mentioned the Magneto Speed chronograph to me a few weeks ago. I haven’t used one personally, but they look interesting. The Magneto Speed works different than most chronographs, in that it watches for changes in the electromagnetic field instead of light like most chronographs. I like the idea of that, because issues related to light cause most of problems for other chronographs.

      I’ve personally tried several chronographs over the years, but really only use an Oehler 35p at this point. I’ve just lost confidence in chronograph readings, but the Oehler 35p is unique because it is a “proof chronograph.” It essentially is two chronographs in one package. It takes two velocity readings on every shot, and automatically compares those numbers to ensure they agree. If they are significantly different, it will alert you that there was likely some kind of error. Knowing that the Oehler is essentially double-checking every shot, I have complete confidence that the velocity it displays is very, very accurate. It is also longer than most chronographs, and the further the distance between the two sensors the more accurate the chronograph is able to be (all other factors being equal). When you are trying to measure strings of shots with standard deviations (SDs) down in the single digits, it’s a joke to think you have that kind of precision with most other chronographs. But the Magneto Speed works differently than most, so I can’t say how it compares.

      Another thing to keep in mind is the Oehler is $575, which is $200-300 more than the Magneto Speed chronographs. It is considered a “Professional Grade” chronograph, and the price tag reflects that. So they are targeting/serving a different buyer.

      I’ve thought about doing a data-driven chronograph field test at some point to really empirically test how accurate different chronographs are. I essentially envisioned lining up several chronographs so each shot would pass over all of them and recording hundreds of measurements and then really analyze the results. You could account for the decrease in velocity as the chronographs got further from the muzzle. I think it would be enlightening. I already have my next field test lined up, but maybe I’ll do that at some point. I’ll be sure to include the Magneto Speed if/when I do. I would certainly be interested to see how it compares.

       
  3. Unclenick

    February 23, 2014 at 11:17 am

    The Magnetospeed appears to use sensor technology like that of an electric guitar pickup: a coil of wire wrapped around a permanent magnet. When a conductor (the bullet) passes close by, since it is moving with respect to the magnetic field lines, those lines induce current flow in the conductor. That current has its own opposing magnetic field associated with it, and the interaction of the induced current magnetic field with the permanent magnet field distorts the latter, exposing the wire coil to moving field lines. Those induce current in the coil, which is detected as a signal by the chronograph electronics.

    There are several things to keep in mind about this approach. One is that a steel-jacketed surplus bullet, which is not only a conductor, but is also magnetizable, will distort the permanent magnet fields a lot more than a jacketed bullet does. A lead bullet, which is a poorer conductor than copper, will distort the fields less than a copper solid because lower levels of current are induced in its greater resistance. Variation in velocity will also affect the size of the signal. Bullet diameter will also affect how much metal is in close proximity to the field. The Magnetospeed makers have developed different sensitivities and different trigger signal profiles to adjust adequately for these factors, but you do have to remember to select the right settings for your bullet to get good readings. You also need to record the settings you used to get future comparisons using the same bullet.

    The Magnetospeed units mount on the front end of a rifle barrel. Just as a barrel tuner affects rifle point of impact and group size, having the unit hanging off a rifle muzzle can affect accuracy, cause stringing, etc. For this reason, you have to work up accuracy loads and test their velocity separately, whereas an optical chronograph lets you do both simultaneously.

    As Harold Vaughn demonstrated in his book, Rifle Accuracy Facts, bullets can pick up in the neighborhood of 3% of their final velocity from muzzle blast hosing against the bullet base after it clears the muzzle. The military has found that post-muzzle effects (acceleration, extra yaw, etc.) are usually complete in the first dozen calibers of post-muzzle travel. That dozen calibers is inside the first Magnetospeed sensor spacing on some guns and calibers. This means the unit will measure some portion of that last 3% of final velocity, but possibly not all of it. It depends on how far the first sensor is from the muzzle. It also means you will change that portion if the unit is not positioned to keep the first sensors exactly the same number of inches from the muzzle each time you use it. It means that if I owned one, I would want to check how results compared to an optical chronograph set up so the start and stop screen midpoint was at 15 feet from the muzzle. The SAAMI standard velocity measuring system is optical and has its midpoint 15 feet from the muzzle. Since that is what commercial ammunition makers use, and I think it is best for comparisons for that reason. Note that the military often uses 78 feet from the muzzle and you need to allow for that for comparison purposes.

     
    • Cal

      February 23, 2014 at 12:42 pm

      Wow. That is some GREAT information. Thanks for sharing!

       

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

 
Follow

Get every new post delivered to your Inbox.

Join 1,018 other followers

%d bloggers like this: