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Oehler 35 Chronograph Testing and Evaluation
by Glenn Morgon

 

  I contemplated adding a chronograph to my inventory of shooting equipment shortly after I started reloading.  I wanted to know just how fast the bullets I was loading were actually flying and what trajectory to expect from those rounds.  The time had finally come to make the investment in a chronograph after building an AR-15 with a 16" barrel I intended to use for coyote, fox, and bobcat hunting.  I knew that with the short barrel the velocities were going to be lower than those listed in the reloading publications but it was unknown by how much.  After taking some time to inspect what was available on the market I ordered an Oehler 35 chronograph direct from Oehler Research in May 2003.  A few days after the order was placed UPS delivered the package to my door.
  With the assistance from my friend Lance we set out to the desert to test the Oehler chronograph. The weather in Las Vegas had started to heat up considerably so we made plans to meet at about sunrise when the winds were forecasted to be down and before things got too hot.    After setting up the shooting bench and the chronograph we fired a few test rounds to check that the chronograph was seeing the bullets and measuring their velocities.  I was initially concerned that testing might not be possible with the sun still low on the horizon and casting long shadows.  However, once we got everything setup we were able to measure the velocities on some cheap fodder bullets placed across the skyscreens to verify the chronograph was functioning and everything was positioned properly.  With the setup completed and the operation of the chronograph confirmed we were ready to get the evaluation of the Oehler 35 chronograph and some hand loads I was developing for my AR-15 underway.  

  One thing I found out very quickly is that a 16" barreled AR comes nowhere close to what the load books list for an expected velocity range. Granted the load book refers to a 22" test barrel, but it was surprising to see where my loads were coming in at. For the load I was using, 3200 fps was listed for the powder charge used whereas I was getting about 2875 fps.

  Throughout the testing, the Oehler 35 was able to grab velocities of our projectiles without any problems.  The chronograph had no problem reading fairly rapid fire, even from subsonic rounds (about 900 fps) at a distance of about 5 feet from the muzzle. I simply held my .44 such that I wouldn't hit the skyscreens, diffusers, or side-rails and emptied the cylinder as quick as I could simply sighting along the barrel between the side-rails. All 6 shots registered. Many times chronographs have a difficult time with subsonic rounds because the sonic shockwave triggers the skyscreen sensors before the bullet actually arrives.  To prevent this a piece of plywood with a hole for shooting through can be placed in front of the skyscreens.  The plywood will block the sonic wave from reaching the skyscreens and causing false readings, however, during our use we found no need for it.  The Oehler 35 was doing an excellent job of grabbing velocities of everything we threw over it.  Until we tried firing Lance's NEF .17 HMR.
    When we tried launching a few rounds of Lance's .17 HMR across it we got no reading. We tried it a couple more times. Nothing. We tried again with the diffusers removed from the skyscreens. Nada. We tried blackening a bullet with a black marker. And again we got nothing. Finally Lance figured out, and I remember reading about it in the manual but forgot, that the smaller caliber bullets need to be fired a little closer to the skyscreen sensors than the bigger stuff we had been firing--.223, .357, and .44.  Once we realized that, the Oehler 35 had no problem picking up the velocities of those minute .17 caliber bullets.  I was surprised to see those little 17 HMRs had some zip to them, about 2650 fps from Lance's rifle.

    The shot string summary of the Oehler 35 I found to be very handy. It reports the high, low, average, extreme spread, and standard deviation.  While the extreme spread and average velocity of a series of velocities are easy to compute by hand, standard deviation can be a bear--just ask anybody who has taken a statistics course.  Even though standard deviation is inconvenient to calculate it is a very valuable piece of data as it tells the shooter how consistent the velocities are.  Thankfully the Oehler 35 automatically handles the computation of standard deviation based on the velocities it measured and has stored into memory.

    Many chronographs use two skyscreens. The Oehler 35 utilizes three skyscreens. Essentially it combines two chronographs in one to verify the readings are correct and not errors induced from muzzle blast or other sources. Oehler calls this third skyscreen a "proof channel."  The function of the proof channel is to validate the measurement taken by the chronograph by comparing the reading with the first and last skyscreens with that of the first and middle skyscreens. If the value falls outside a specific range (range varies with velocity) the display flashes to let the shooter know the reading is suspect and the shooter can choose to omit the value from memory. Or, if a printer is attached, an asterisk will be printed next to the suspect reading. In a standard chronograph with two skyscreens it can be difficult to spot read errors. In some cases, erroneous readings without a proof channel are obvious. If a shot string reports 1000, 1025, 1017 fps, then one registers 2000 fps with no noticeable difference in the firing, the chronograph read error is obvious. However, smaller errors may not be obvious. If the fourth reading was 1120 fps, could it be an error or just a bad load?  With the proof channel, the Oehler 35 will automatically determine the validity of the reading.

    While we didn't observe any erroneous reading reports (the flashing display) we did notice some .38 Special hand loads we were shooting had velocities all over the place. The low was around 800 fps, with the high at around 950 fps. The chronograph removed doubts that the velocities may have been read in error as the display was in a steady, non-flashing state. This told us that there was a problem with the consistency of the loads and not errors in measuring the velocities.

    One of the features I like with the Oehler 35 is the power switch, or rather the lack thereof. The unit is powered on by simply plugging in the first skyscreen. The unit is powered off by unplugging that skyscreen. There is no switch to accidentally bump and power off the unit and the plugs are held securely in place so there is little possibility they will fall loose on their own. In addition, there is little possibility of leaving the unit turned on when it is packed up since the skyscreens are likely to be unplugged when the equipment is put away.  The Oehler 35 also features a compartment for a spare battery.  So should the battery fail, a spare is easily locatable.

    The Oehler 35 is very easy to use.  The unit needs only three buttons to operate.  Pressing the first button allows the shooter to enter the edit mode for the measure velocities of that group, or by holding the button down the chronograph will reset.  The second button permits the shooter to omit a velocity from the summary, or remove the last fired shot from memory should the proof channel detect and error.  The third button presents the shooter with the summary calculated from the measurements in memory.   

    My chief complaint with the Oehler 35 is the wiring to the skyscreens. The cables to connect the skyscreens are generously long to allow for versatility in how the shooter chooses to space the skyscreens and how far out from the muzzle to place the skyscreens.  Unfortunately, I found myself stepping on the cables or tripping on them if I got my feet too close to them.  Now I could have alleviated the problem myself by bundling the cables instead of leaving them loose on the ground, but I found it easy to simply keep my feet away from them.  For future use I will add some reusable cable ties to address this problem.  

    One of the upsides to using a chronograph that has skyscreens separate of the electronics is the protection the electronics of the chronograph have by not being in front of the gun.  Many long-time chronograph users have told me that at some point a skyscreen will wind up with a bullet hole in it.  In the case of the Oehler 35, if a skyscreen winds up with a hole in it, the skyscreen can be replaced or rebuilt at a minimal cost.  The side-bars and diffusers that the Oehler 35 uses are made of plastic and require no tools to mount.  They are very durable and stable in the wind.  An advantage they have with being plastic instead of metal is that they will not bend should the tripod fall over or succumb to gravity in other ways.

    The only thing I had to provide to make the Oehler 35 functional was a tripod for mounting the skyscreens on.  I obtained an inexpensive ($20) camera tripod that did the job, however, I would prefer a sturdier tripod for future use.  The tripod would not have been necessary if I ordered the 4 foot skyscreen spacing kit from Oehler as they include two stands for supporting the skyscreens in that kit.  

    So after having experimented with the Oehler 35 chronograph I started to ask myself if I really felt that it is a necessary piece of equipment for the average hand loader. The conclusion I came to is possibly. Basically a chronograph is another tool in the shooter's toolbox. When looking for the cause of large groups it is nice to have a reference on how consistent the velocities are; especially when firing rounds at 100 yards where even rounds that show a lot of velocity differences may group nicely. But, those same rounds when fired at 200 yards or farther will probably be are all over the paper. If the groups are large and the velocity differences/standard deviation is large, it helps clue a loader in to where the problem might be. The chronograph also provides the loader information on what can be expected from his loads when reaching out to the longer ranges. Knowing the trajectory is necessary in predicting where the bullet will land at a given range and to know the trajectory the velocity must be known. In addition, it provides a general idea as to the maximum effective range a cartridge may have for ideal terminal effects. A quick comparison shows that the impact force of the .223 loads I worked up have the same force at 525 yards as the 17 HMR rounds do at the muzzle when fired from my particular test rifle. Knowing this, and how ineffective the 17 HMR has proven to be on coyotes at even relatively close ranges, would cause me to reconsider any long distance shots, even if the wind is calm, as the on-target energy is lacking.

  Specifications of the Oehler 35 chronograph

Oehler Research is located in Texas and manufactures their chronographs in the USA. Their contact information is:
    Oehler Research, Inc.
    PO Box 9135
    Austin, TX 78766
    Phone: (512) 327-6900 or (800) 531-5125
    Web: www.oehler-research.com

For questions or comments about this review, please feel free to contact me at gvmorgon@earthlink.net


The setup of the Oehler 35.  On the right side of the bench is the chronograph with the wires leading out to the skyscreens.


The skyscreens mounted on a tripod.

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