Chrono Question

[fishnbob]

I set my shooting bench up in the shade of some big oak trees. I set my chrony about 15' from the end to avoid the muzzle blast and out from under the trees. Well, over time, I notice the limbs growing out and somewhat over my crony set up. I buried some PVC pipes in the ground and concreted around them to keep them in place and put sand in the pipes so that all I have to do is set the tripod in the pipes, raise it up to the marks and I'm all set up, right height and lined up with the bench & target. Question: Do you think the presence of high limbs may be affecting my readings? I would have to cut the whole tree as the limbs are way up in the air but I can't tell if they are directly over the chrony or not. My readings sometimes vary 25 fps. Is that a normal variance? Moving the setup is not an easy option. What would putting the diffusers on do? :???:

[Mal Paso]

25 fps shot to shot is reasonable.

I think the only problem might be if the tree shaded the chronograph so that it did not read at all or read sporadically.

[Larry Gibson]

If you mean your reading of the same ammo velocity averages vary 25 fps day to day then that is well within the accepted norm. Even on the same day a varience of 25 fps between different shot strings is acceptable. The larger the string of shots the less the varience will be. a 3 shot string only gets you an idea, a 5 shot string gets you closer but a 7 -or 8 shot string is the minimum for reliable information. A 10 shot string is industry standard and is accepted as 95% reliable indicator of that lot of ammuntions performance.

Larry Gibson

[6.5 mike]

Thanks Larry, I always wondered how many shots were needed to have a good data.