Originally Posted by neilL
Momentum (vector) vs kinetic energy. mv vs 1/2mv^^2 (scalar)
but if you rely on momentum you end up questioning chrono readings - we shoot with the butt held in the shoulder which reduces any reverse movement of the rifle but when using the chrono everyone just holds the rifle loosely (relatively) which lets the rifle move backwards. Upshot is that the pellet velocity should be lower over the chrono than it would be when holding seated, etc.
Pellets may splat but they almost always bounce off the target (easily seen) so they do not entirely transfer their momentum to the target.
You need to use something you can measure independently of how the energy is transferred.
Anyway - something like that and I really did't want to start a chrono thread :-)
Ah right, I probably didn't explain myself very well then.
Ok... there's two 'calibrations' required here.
1. Set up 1 device so that it produces the same results as a 5fpe pistol would do.
2. Set up X additional devices to be the same as the first device. (the reference device)
For calibration 1, as you've said just figuring out a method to make sure the device has 5fpe of kinetic energy is wrong. Squashed pellets, bouncing pellets, momentum impulse theorem, elastic / inelastic collisions yada yada..
For calibration 2 though (which is what I was talking about regarding measuring momentum) I think it's probably sufficient to make sure that X additional devices has either the same Momentum or Kinetic Energy as the reference device to make sure they're calibrated to perform about the same as the reference device.
Now you could say - why do we need two? Why not just set every device up using whatever method you used to calibrate the 1st reference device.
My thinking is, that trying to calculate what's needed is going to be impossible because there are too many unknown variables. Possibly a better way to calibrate is by trial and error (over a long period of testing).
We don't want to be going through all of those trial and error tests for every device, so we need a way to take a benchmark measurement from that reference device and use that to set up the others.
It may well be that Pete D has already done this - it might be that he did some testing and found that if he set up the plunger to give about 1fpe of kinetic energy, that it was roughly equivalent in terms of collision as an air pistol putting out 5fpe at the muzzle.
(Still don't know if I've explained that any better, hopefully it'll make sense to someone)