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Old 11th November 2014, 12:56 PM
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RobF RobF is offline
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Originally Posted by neilL View Post
Why are you concerned about Gravity? You only measure mass and velocity. No force needed and no measure of acceleration or deceleration.
Pellet drops in flight of course.
Actually nobody weighs pellets accurately anyway. If it becomes an issue you need a set of BFTA weights equivalent to, say, 10 7.9s or 8.4s. Then you make a reasonable average for 10 over any slight variation between pellets.

Balance scales are always better than anything based on a load cell ( strain gauge ) but it is highly unlikely that a course would ever be up to a laboratory standard.
With SI units Neil, gravity is removed, because we deal with mass. Tis a good thing. But being english, we're still measuring using potatoes and weight, not mass with the old 12ft-lb formula... and that's where the gravity bit comes in... wieght = force = mass x acceleration (gravity)

Muzzle Energy = weight x velocity squared / 7000 (the amount of grains in a lb) by gravity x 2

Often the "7000 (the amount of grains in a lb) by gravity x 2" bit is replaced by just a number say 450240... it entirely depends on where you get your muzzle energy calculator formula from... some use 450450, some use 450236, some dont state it.

For an 8.4gn pellet,

5376000 / 450240 = 11.940

5376000 / 450450 = 11.935

Not a massive difference granted, but if you're going to say just use one decimal place, then the rounding error could see you 11.9 or 12...

The margin of difference is about 2-3 fps between those constants... or in other words, 50% of what the BFTA for instance use as a the deciding limit.

Working with SI, we lose the weight calculation of the formula, and there's no constant to worry about. I've got no idea what chronos use as a constant either, but the working practice is to just measure the velocity. So I was just trying to work out what figure to use for a set of pellet weights we could stick on the CSFTA chrono, so that when someone comes along with a 7.6gn pellet, we know what they should be flying at, and that figure isn't upset by Holly's wallet bending the earth's gravitation field or us being at one end of the region to the other, as gravity varies by latitude as well...

I've got no idea what the law uses. So we'll use 16.270 Joules, as at least the online calculators agree that's 12.0001 ft-lbs... then minus the 5fps.

BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep
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