mrbadexample wrote:...actually it is derived from a simple ballistics calculation.
mass (in grains) x velocity(fps) x velocity(fps)/450,436=ft/lb there are 437.5 grains in a standard ounce. (i've been shooting real guns for about 30 years)
uh, that's just the KE=(1/2)mv<sup>2</sup>, which was posted several posts ago.
If you have a chrony (or other method of measuring muzzle velocity) then you don't need to fiddle with estimates based on volume, fuel etc. What Ragnarok was getting at was that with a bit of experience with the mathematics you can estimate the performance of the gun before you even build it.
... i could probably tote out a shooting chrony, but i just use my computer and video editing software and shoot at a target at a known distance. the computer breaks down the video to thousandths of a second.
Standard video is 30 (actually 29.97) frames/second, so yes, thousandths of seconds as in +/- 0.033 second. To get a reasonably accurate velocity, say +/- 10%, the round has to be in flight for at least 10 frames or 0.33 second. The velocity calculated over that time is the average velocity, not the muzzle velocity. Without the muzzle velocity you can't calculate the muzzle energy. Some ammo won't slow down too much in that time, most ammo will slow down pretty significantly in the first 0.33 seconds of flight. To get +/-5% you need twice as many frames and you get a resulting velocity that is even farther from the muzzle velocity.
To get +/-1% you would have to have the round in flight for 3.3 seconds, the calculated average velocity wouldn't be anywhere near the actual muzzle velocity. Even a high powered rifle round will have slowed down by a huge amount after 3.3 seconds of flight.