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40 fps difference IR versus normal screens?


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I recently bought a CED M2. I didn't spring for the IR screens because I mostly would be testing outdoors in nice weather. I've been working up a load for my JPL 6" gun with MG 165gr CMJ's. Today a couple friends (also with a CED M2) were wanting to chrono with their equipment when I finished with the bay. Just for interest, they had me plug my M2 into their IR screens. Same ammo and to my surprise on a 10 shot string the average was about 40fps different (1013fps versus 971fps...higher with the IR screens).

The "good news" is that my screens were reading slower not faster - therefore if I work up a load on my screens averaging 1020fps I should be more than safe at a bug match. Anyone have experience with side by side, same day testing IR versus non-IR?

~Mitch

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Ants,

Here's the data. Same gun, same ammo, 15 minutes apart. MG 165 CMJ, 1.18" COL, Wolff primer, 5.2gr Ramshot Competition.

My screens:

String 4 995.7 965.9 952.3 994 975.2 967.3 965.2 972.8 961.8 962.3

High: Low: E.S.: Ave.: S.D.: 95%:

995.7 952.3 43.4 971.2 13.2 10.5

Richard's IR screens:

String 5 994.9 1028 1007 1000 1002 1003 1046 1024 1009

High: Low: E.S.: Ave.: S.D.: 95%:

1046 994.9 51.1 1012.7 15.6 13.5

Seems like slightly more variance in ES and SD with the IR screens, but within a few fps. So it doesn't look totally like the difference is just random statistical error. Ultimately the only velocity that matters is the velocity that the chrono reads at a match, so it has me wondering...which would be most like the chrono at a match?

~Mitch

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What you experienced is not unusual. Different lighting condition will have an effect on your readings.

Greg L. (I believe is the Chronoman) has written about chronos. Even at Nationals when using a light box, the two chronos often post different velocities throughout the string.

If you are that concerned about being as close to the match chrono ( Nationals) here is what I have done. I have National and Area 2 Ammo and chrono sheets for about five years now. I use that information as a control for when I am working on my next batch of match ammo. I just try to get the two close to the same readings. I never expect them to be exact, too many variables in the testing.

I try to follow some advice that Mike Voight told me; do the minimum plus 5 and you never have to worry.

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I try to follow some advice that Mike Voight told me; do the minimum plus 5 and you never have to worry.

I should chrono my open gun loads, which have at Area matches consistently come in at 172. If it reads 172 on my chrono than I can be confident. If it comes out under or over I know there's some adjustment factor.

Fascinating that at a big match with double screens in a box they give different readings. I'm guessing not different by "that much". Do they take the higher of the two?

~Mitch

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