How to determine Foot poundage.

[harry mac]

I think the British, as distinct from the American Standard for g results in a divisor of 450436.68 .

The 5lb cannon ball (and who ever used a 5 -pounder? ) won't have the same trajectory as any airgun pellet because there's a helluva lot more grains per square unit of cross section in the 5lb ball, so it'll retain velocity much better. You'd probably start to notice the difference within 50 yards or so.

Mik, for as long as I can remember, British publications have put the devisor at 450240. American sources I've seen all quote 450400. I don't know why there's a fifference.
Do we know what figure the police/phorensic service use when/if they have to test an airgun?

[Turnup]

Mik, for as long as I can remember, British publications have put the devisor at 450240. American sources I've seen all quote 450400. I don't know why there's a fifference.
Do we know what figure the police/phorensic service use when/if they have to test an airgun?

The divisor is the result of the acceleration from gravity ("g") and the number of grains in a pound, and the constant "2". The number of grains in a pound and the value "2" are fixed values. The differences arise from small differences in the value assumed for "g". "g" is not a constant all over the surface of the earth, it varies with altitude, underlying geology, and proximity to the equator, so any value applied universally will likely be an approximation to the value of "g" at your exact location. For perfect accuracy the forensic service would need to ascertain the exact value of "g" at the test location and adjust the calculation accordingly, but the differences attributable to variations in local "g" are very small (approximately .03% between the values you quote) and much smaller than the other measurement errors. To be credible, a forensic result for muzzle energy would also have to specify the maximum error in the given figure, e.g. 11.98 ft-lbf +_ 0.1%

[ballisticboy]

The divisor is the result of the acceleration from gravity ("g") and the number of grains in a pound, and the constant "2". The number of grains in a pound and the value "2" are fixed values. The differences arise from small differences in the value assumed for "g". "g" is not a constant all over the surface of the earth, it varies with altitude, underlying geology, and proximity to the equator, so any value applied universally will likely be an approximation to the value of "g" at your exact location. For perfect accuracy the forensic service would need to ascertain the exact value of "g" at the test location and adjust the calculation accordingly, but the differences attributable to variations in local "g" are very small (approximately .03% between the values you quote) and much smaller than the other measurement errors. To be credible, a forensic result for muzzle energy would also have to specify the maximum error in the given figure, e.g. 11.98 ft-lbf +_ 0.1%

According to the latest published survey results, using the average value of "G" for the UK the constant should be approximately 450750 (actual value is about 450753 ish). If you are depending on the differences in all the values to prove your gun is legal you are probably not going to manage it. The 450240 figure usually quoted seems to be accurate in Salt Lake city.

[Turnup]

Been thinking on a bit about this. If you have a given pellet, it has a given mass, the same everywhere regardless of the local value for g.

If you use a spring balance to weight it, then the measured weight will vary with g.

If you use a balance scale, the weight you measure will be the same everywhere, regardless of the local value for g (assuming that the local value for g is the same for all parts of the balance, which I think is pretty safe). It would therefore be possible to change the scale of the balance to directly read mass rather than weight.

So, it should be possible to measure mass independent of the value of g, using a balance scale. In fact I can envisage a means of measuring mass using a spring resonance method, which would also give mass independent of g.