I have a Chrono Chronograph, been using it for years now.
A chronograph provides useful data but is not to be trusted implicitly. There are things such as instrumentation error. I believe this is due to misalignment. When you consider the short distance between the sensors, around 12 inches, and velocity is calculated based on the time it takes the bullet to cross that distance, the tiniest variation in that distance caused by off axis misalignment will create differences in reported velocities. I have found over the years that values can vary day to day. Velocity averages will never be exact, but I believe they are comparable as long as they are within the Standard Deviation of a previous session,. I believe that you should start off a chronograph session with a “calibration load”. For me this load is something that I have fired before and have confidence in the velocity numbers.
I stick it on a cheap camera tripod that I purchased from Walmart. I level my chronograph using a cheap Laser Level purchased for under ten bucks at Harbor Freight Tools.
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=54050. I use this to level the Chronograph and use the laser to orientate the center line of the chronograph on my bench rest. I believe this reduces my misalignment error. I have noticed the slightest bump or movement in the chronograph will give radically different velocity readings.
Sometimes powder chunks going over the sensors create warp speed readings and the only cure is to move the chronograph further out from the muzzle. I do not think it makes any real difference on indicated velocities. Black powder rifles blow out so much unburnt chunks of powder, that I have had to put my Chrono out maybe 30 feet away or I get display errors. I have a M1911 Swiss that with its long barrel and slow burning powders require the chronograph to be placed out further than normal or the screens get blown off..
Acceptable velocities depends on the loads and the rifles. You will find some barrels are faster than others. And as you pop primers in the fast barrels, you will figure out the thresholds for that firearm.
What about those funny numbers , Standard Deviations and Extreme Spreads? The Standard Deviation and Extreme Spread numbers are not as important as what your group looks like on paper. But for those who want a standard, assuming already outstanding on paper performance, I am going to give you some impossible criteria. I asked the gunsmith who barrels my Match Bolt Rifles, this man is Nationally ranked Highpower Competitor, last couple years was in the upper ten, has won the Long Range Aggregate and I think Wimbleton, about his criteria. He wants a long range load that has a SD less than 10 and a ES less than 20. Of all my loads that I have chronographed, and there are many, I have had two or three that met this criteria. But there is a standard for you to shoot at. Pun intended.
The chronograph is a tool, useful for checking velocities. Consistent ammunition is always good, means you have your reloading processes down. But the numbers do not guarantee accuracy. You will find that errors in position, errors due to wind changes, errors due to flinching, are typically larger than your ammunition errors. But it is worth making good enough ammunition that you can concentrate on fixing the other things.
Oops.Well, I got replacement sky screen wires, but shelved the unit in favor of making the most accurate loads I could.
