I found it helpful to put a wrap of tape around each plug. Green for "Start", yellow for "Mid" and red for "Stop".
If you want downrange measurements, place something under the back of the box so that it sits upright. You can then read the display thru a spotting scope. Oh, put the box as far from the target as the cables allow. Skyscreens, rails and diffusers are inexpensive and easy to replace. That "brain", not so much.
I became fascinated with chronographs as soon as I began to obtain firearms (teenager). A guy had an Oehler Model 10 at the range. This system used a piece of paper for each screen. The paper had silver printing on it and a bullet passing thru tripped the counter. A new piece of paper had to be used, on each screen, for each shot. You then turned a dial that corresponded with some numbers and you wrote down whatever number the dial was on when the needle pointed to "Yes". You then looked that number up in a table and it would tell you the velocity of that shot.
I wanted one. I contacted Oehler and found out they had a new model, the 12, that used "skyscreens". That's right, no more paper screens. I saved up my money and bought one. I still have some data from that chronograph. While in college, I sold it to two friends. Much later, I tried to buy it back. Each friend insisted that the other one had it. I suspect it has been lost forever.
I met Dr. Oehler in about 1996. I was assigned as a Firearms Instructor at the FBI Academy. I learned he was going to be visiting the Ballistic Research Facility (BRF) and I made sure I got down there to meet him. I told him I was a long time fan and that I had owned a Model 12. His response: "Son, you are not old enough to have owned a Model 12." I believe subsequent conversations have convinced him that I did, indeed, own a Model 12 at one time.
Within less than a year I was notified that I was being assigned to take over the BRF. I had oversight of that facility from April, 1997 to August 2012. Like most in the industry, we used a lot of Oehler equipment. We shot a lot of tests. I'd hazard to say that we got to experience many issues that others may not see or understand. I spent a lot of time talking with, and learning from, Dr. Oehler. during those years. At the BRF, we setup regular "training sessions" wherein we would run multiple measurements and change setups to see how it would effect the outcome. We ran sessions to create issues and troubleshoot them. We got pretty experienced with measuring pressure, velocity, time of flight and terminal performance.
Following retirement, I've been fortunate to become close friends with Dr. Oehler.
While at the FBI, my main focus was the best interest of the U.S. Govt. I didn't care whose equipment we used, provided it was the best. Additionally, if newer, better equipment came along, we would have switched.
Most of what we used was industrial grade (Model 55 and 57 screens, Model 35 BNC, 82, 83 and 85 computers) but we did make extensive use of the Model 35P when shooting at remote locations or outdoors. We also used them for impact velocity measurements downrange.
There was a time when the Model 35P was discontinued. We had 4 or 5 of them and I was concerned about what we would do when they were no longer functional. One problem I found with them was their propensity to jump in front of bullets when trying to measure velocity at longer ranges. I distinctly remember measuring impact velocities of M855 projectiles at approximately 300 yards, launched from a rack-grade M4. Memory is that 15" groups were normal. Not conducive to lifespan of downrange equipment. Yes, we used steel shields but I can still say we shot a few (or more) chronograph parts. Though we were sticklers about not wasting money, testing has costs. If a $600 chronograph is damaged while conducting a $54 Million ammunition procurement, it is just considered a cost of doing business.
I set out on a quest to find a replacement for the 35P. We purchased a few different models. The most promising model had a rail that folded for transport. I set it up in front of an M35P and shot some .308 thru both sets of screens. There was 50 or 60 fps difference. Even though the M35P proof values were very small, out came a tape measure and the Oehler screens were spot on at 4'. The new chronograph was only able to have a 2' spacing and, due to the flexing of the rail, it actually measured 24 1/8".
I contacted the manufacturer and was getting nowhere. He spent a lot of time explaining how chronographs work. He asked for a photo of the setup so I added another set of screens (Model 57 this time) and re-shot the test and sent him a photo showing the M35P agreeing with the M35 BNC (57 screens). However, his chronograph still showed 50-60 fps off. I explained what I thought the problem was and he sent me a new rail. The new rail was exactly the same as the old one. We solved the issue by securing the rail with a piece of angle aluminum. Thereafter the chronograph worked as well as I'd expect any light screen chrono with a 2' spacing to work. Still, it did not have the "Proof" screen so trusting any numbers would require TWO of them to check for agreement.
Yes, we were relieved when Oehler brought the M35P back into production.
After retiring, I built a test range and purchased two of the M35P chronographs (can use one near muzzle and one downrange...but only as far as you are willing to risk the equipment). Yes, I also purchased replacement screens/diffusers (experience speaking!).
Additionally, I've been fortunate in Dr. Oehler allowing me to follow along and assist with his creation of the System '88 and System '89. I've used the System '88 a good bit and am looking forward to the System '89.
It has long been understood that velocity can be expected to be unique to particular systems. It is about time that the community understand and recognize that drag (BC) should also be expected to be unique to particular systems. In other words:
"You don't get your velocity off the box of ammunition, why would you get your BC out of the bullet catalog?"
