SpudFiles

boyntonstu wrote:2 pieces of paper spaced 10 feet apart would measure 1,000 fps if the 2 sound spikes were 1 millisecond apart.

IOW If your sound resolution was limited to 0.001 secon the paper spacing would have to be increased to let's say 20 feet.

Typical sound card max sample rate is 48KHz, that's 0.000021 seconds (21uSec).

If the traps are ten feet apart and the ammo is moving at 1000 FPS then there are (10')(1/1000FPS)(48,000sample/sec)=480 samples. The plus or minus one sample error represents an error of 0.2% (+/-2 FPS in velocity).

Put the paper at a much more practical separation of 1 foot. That gives 48 samples. Error is 2%, +/-20 FPS in velocity at 1000 FPS.

Omit the first piece of paper, strap the mic to the second and use the muzzle report as the first sound signal. With the mic contacting the one piece of paper the impact sound will be quite distinct from the muzzle blast and it's echoes. A bit'o math to correct for the distance from the muzzle to mic.

Larger projectiles work fine with either magnetic pick up coils with a small magnet in the projectile, or a wire break with a sound card. I've used both with good success.

Audacity is great software to measure times of events. Audacity is free.

I have a Oehler 35 P I picked up at a second store (goodwill) for 50$(U.S.).

It does a few more things than a standard chronograph.
so its somewhat overkill for spudguns and airguns but helps with load development when reloading or checking out factory ammunition.

Separate screens and main unit make it cheaper when you blast a screen but make set up a little longer.

I'm using a M-1 master chrony. Same as the F-1, but then puts out m/s and has a detachable wired display.
It works so easy and nice, its unbelievable. Just set it up, turn it on, shoot over it and voila, you have your velocity.