sputnick wrote:Well, DO they need to be exact measurments?
It's nothing that has to be done, but knowing that kind of thing makes it easier to guestimate how much power can be squeezed out of the rifle.
However, yes, if it's being done, it does need to be pretty precise, particularly the wire diameter measurement - which because in the formula it's used to the 4th power, a 10% error in it results in a 46% mistake in the end result.
I'm not sure what you mean about compression.
Compression is both the preload on the spring when in an "uncocked" position (relative to it's totally unloaded length), and the further compression applied on cocking - both distances, not forces.
I don't know the velocity, I do not have a chrony, and I'm a little apprehensive of the hang time system, firing straight into the air goes against everything I have learned about gun safety.
No need to use the hang time method. Much better choice is to get some kind of recording device, set it to record in the middle of your range, fire at something that makes a noise, then take the resulting audio file, open it in something like Audacity, measure time from firing to impact, divide the distance by that number, and you'll get a reasonable estimate.
Then work out the kinetic energy from the velocity and pellet mass.