
General Airgun Chat Chat and banter that doesn't fit anywhere else... 

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#11




To do it correctly the scales need to be beside the chrono and the pellet weighed by the marshal before he inserts it into your barrel
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He who never made a mistake never learned! He who never missed never shot! 
#12





#13




Quote:
http://en.wikipedia.org/wiki/Poundal 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 23 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 ftlbs... then minus the 5fps. Following?
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BFTA/NSRA County Coach CSFTA Chairman/BFTA Rep 
#14




Hmmm, maybe it's time that some of the existing firearms law should be updated and rationalised; now there's a thought.

#15




Interesting post.. for all my inquisitive nature I'd never considered the derivation of the constant. I've unquestioningly used 450240 ever since I first read it in Airgunner () at the age of about 15
Rob; as you point out this issue arises from varying available values for gravitational acceleration; would it not be easiest to work through the SI formula converting to imperial, using the standard gravitational acceleration constant of 9.80665m/s^2..? As you rightly point out the potential for inaccuracy from such variations is pretty small  at this point I'd start worrying more about the accuracy of my measuring equipment, rounding errors etc 
#16




Quote:
That means you'd be using the constant 450436 in your calculations. 800 fps for an 8.4 pellet would be 11.935fpe or 11.99fpe for an 8.44grn pellet. 
#17




Quote:
I did wonder if this standard figure is what Skan chronos use to calculate muzzle energy and was going to test it, although the readout doesn't have enough decimal places to quantify the 0.04% difference in the results 
#18




Quote:
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BFTA/NSRA County Coach CSFTA Chairman/BFTA Rep 
#19




Quote:
((450436450240)/450436)*100 = (196/450436)*100 = 0.0004*100 = 0.040%. Using this method applied to the 5ft/s within 800ft/s, I get: (5/800)*100 = 0.0062*100 = 0.62%.. which sounds about right as 8ft/s would obviously be 1/100th or 1%. To illustrate the influence of using different conversion factors, let's say we have an 8.4gn pellet travelling at 785ft/s. Calculated using the smaller, traditional constant of 450240 gives an energy value of 11.4967ftlb. Recalculating using the constant 450436 gives an energy value of 11.4917ftlb; carrying through that 0.040% difference from the constant to the energy. I've just had a look at the Skan and the energy value reads to 3 decimal places; so actually it should be possible to determine which constant (if either) it uses. A test shot with an 8.40gn pellet gave a velocity of 792.07ft/s and an energy of 11.704ftlb. Repeating the calculations above for the new velocity gives an energy value of 11.7047ftlb for the constant 450240, and 11.6996ftlb for the constant 450436; so it appears that the Skan uses the more traditional value of 450240.. despite it not really being aligned with the "standard gravity" value. Ultimately then, it would seem prudent to use this value; since I believe most clubs and polis forces use Skan chronos; and using the larger constant will result in a lower value than using the higher one  so you could end up in a spot of bother if your gun is doing 11.99999ftlb calculated using the smaller constant.. 
#20




Wait a second. I'm a slow thinker from the european mainland.
Do you mean, that grain is a force, equivalent to newton and not a mass that is equivalent to gram? 
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