The Oehler P35 Chronograph is my favorite go-fast gadget. I have always liked gadgets (calculators, then computers and electronic gizmos), and fast things (fast cars, high velocity rifles, fast computers)–the list never stops. So naturally, when Oehler first made affordable chronographs available to the public, I had to have one. I bought my first Oehler Chronograph about 20 years ago when light screens first hit the market. A chronograph is a simple tool and only does one thing. It measures the velocity of a bullet. From that one measurement a wealth of information can be gained by a hunter or shooter if you know how to use the information. We’ll look at how the P35 works and try to explain how the ODHA uses the information to develop loads and improve our shooting.
The P35 comes with three light screens, three defusers, a bar to hold the light screens, and an optional printer. As you have probably already figured, the light screens are set up in front of your shooting bench on the bar so you can shoot “through” them. Actually, you shoot through the beam of light that is projected up from the screen toward the top to the orange defuser. When the bullet cuts through the first beam, it starts the clock in the chronograph, and when it passes through the second screen the time is recorded. When it passes through the third screen, the time is again recorded and the two measurements are compared for consistency. The times are not averaged, they are only used to prove the measurement has been taken correctly. If sun glare or some other problem occurs at the shot, the P35 will reject the reading, rather than give you a guesstimate.
So what can you do with the info? First of all you will know the exact velocity of your ammo and can zero your rifle properly. Every rifle is different and every batch of ammo is different. Velocity can vary up to 300 feet per second (fps) or more. One of the most popular rifle calibers used is the 3006. Most ammo in that caliber is 250 fps slower than advertised, and hand loads can’t do much better. When the ammo is going 250 fps slower than you think it is, hitting a deer at 300 yards becomes a guessing game. If you know the true velocity, you can zero your rifle properly.
Is there still enough power there to get the job done when velocity is 300 fps slower than it should be? That’s the next thing you can figure out with the P35. If you know the true velocity of your bullet and the weight, you can calculate the power at any given range. Plug the figures into the JBM web page and it will give you energy at any desired range. You’ll need the ballistic coefficient for the calculation. Most hunting bullets are around 0.350 to 0.400. You can use 0.375 to get a close estimate or you can get it from loading books or on-line data. Sierra Bullets has a complete listing on-line.
How much is that bullet dropping at 300 yards? That also can be obtained using the JBM calculator. It will print the trajectory at any desired yardage interval. Just plug in the numbers and read it off.
If you are a hand loader, the chronograph is your best friend. It keeps records of your series of shots and gives you the standard deviation between shots, the average velocity, and the highest and lowest velocity of the series. How does this help? Consistency in velocity means accuracy. When you shoot five shots of a given load and the velocities of all the shots are very close, you can almost bet the load will also be very accurate as well.
You can also use the P35 to help you develop max loads safely. Notice I said help!! Nothing can replace good, safe loading practices and common sense while loading ammo. But, as pressure increases, velocity increases (up to a point). When your velocities start approaching maximum associated with the particular load you are developing, you can bet the pressure is getting to the max also. The chrono will also teach you that more powder does not always mean higher velocities. Only so much powder can be burned in the barrel and you may even see velocities fall off or get very inconsistent when the powder charge is increased.
My P35 has been a great aid to me for many years. It has performed perfectly for me and the battery will last the average shooter about five years. Those orange defusers seem to work very well in all natural light conditions. I have never tried using the P35 indoors. I know that it will not work with florescent lighting because the florescent bulbs blink continuously. This blinking triggers the calculator so you will have to try incandescent lights if you use the chrono inside.
The complete Oehler P3 investment for hunting club, shooting club, or the individual shooting enthusiast.
If you want to see a grown man cry, just clock the velocity of his ’06!!
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