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Old 12th November 2014, 12:42 PM
cloverleaf cloverleaf is offline
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Join Date: Sep 2014
Location: Oxfordshire
Posts: 117

Originally Posted by RobF View Post
Think it's less than that, but 5 fps at 800 is 0.006%, so accuracy is important, especially as a lot of chronos round, and we round ft-lb.
For the conversion constant I arrived at the difference between 450240 and 450436 as a percentage of the larger number in the following way:

((450436-450240)/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..
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