Re: Is my chronograph off?
100fps isn't TOO much... Atmospheric conditions can affect it a lot. Usually hot temperatures increase velocity, sometimes to dangerous levels if you load and test max loads in winter and use them on a hot day later on. Some powders are better temp. stable than others too. KB's have happened this way with less stable powders and large temp. shifts combined with max loads (especially in .40 pistol, it is very sensitive to any changes in pressure).
Also if your barrel is hot, you could be firing what are essentially hotter loads if they sit in the chamber long enough to warm up.
Elevation and air pressure changes coupled with temperature changes can definitely affect it, as to how much, I'm not sure myself. I suppose you could work it out on paper, but I'm too lazy to do it now.
Distance to your chrono can cause variations in readings too. FBI sometimes uses a shield that prevents muzzle blast from affecting the data, basically a hole cut in wood in front of the chrono that lets the bullet through but not much blast. And it is possible your chrono is in fact off. You may could try changing batteries too if you think that may be an issue.
Also, how precise were your loads? It isn't uncommon to have some variance of tens of fps between loads, particularly with lesser components or components that haven't been weighed. Or rounds loaded on a progressive press. If consistency is key, It may behoove you to weigh each bullet and even brass, and separate them according to exact weights. Weigh each powder load, I use a digital dispenser and now and again check it against a blance beam. Takes time, but MMU does this, and I suspect the AMU does too. They claim superb results and consitency, and at the ranges you are speaking of, every little bit counts.
Hope this helps, it is what comes to mind immediately, I'm sure others will have some other good tips to chime in too. Great place about this site if you ask me.
PS, clean your chrono too, the IR sensors if you have them, whatever. Just wipe it down. Also note some chronos are VERY light sensitive, finicky to the nth degree, mine is, so keep that in mind. Also keep in mind the chrono measures the speed of the bullet and the programs measure the weight and volumetric data of the load plus other variables --it is a good estimate, but the chrono is the ultimate test, and barring problems with the tolerance of the chrono, I'd take the chrono's word over a ballistic program anyday.
Good luck, safe shooting!
