Help determing FPS?

[vng21092]

Hey guys, I just got finished putting together my 4.5mm x 24'' barrel and decided to test out the FPS. According to GGDT, I should be in the 950+ FPS, (994 to be exact) but of course I'm skeptical, so I decided to try out the ghetto chrono using a mic and analyzing the audio. If it's not too bothersome, please download it here, and tell me what you think the FPS is based on your analysis. All three shots were taken at 45 feet. I used Audacity to try and figure out the time between muzzle and hit, but if you guys have some better tool, feel free. Thanks

[Gippeto]

Not going to bother.

Here's a better tool...and it's free.

http://www.chronoconnect.com/mobile.html

[vng21092]

oh... You know, I had something similar to this before, but it had issues because it never picked up the sound of impact, but I'll check it out, thanks. Have you tested the app out yourself? If so, how accurate would you say it is?

[Ragnarok]

Taking a look myself in Audacity, I'm getting timings of probably either ~78ms or ~87ms.

Not having actually seen the thing fire, it's hard to know what if I'm hearing the sound of the pilot or the main report; however, the waveform has some pretty distinct transitions that are fairly easy to pick out.
My guess it's that it's 78ms from muzzle to target, and the other 9 ms is the pilot venting.

That equates to about 175m/s, or 580 fps, as an average over that distance. However, 4.4mm BBs aren't hugely aerodynamic, so just to do an estimate of how much it would show down over that distance...

This is going to be a crude estimate, as the Cd of spherical projectiles changes fast in the region of Mach 0.5 (vaguely where we're at) and I'm having to ignore the effects that the muzzle blast may be having on drag, but the specific drag area of a 4.4mm steel BB is about 1.3 x 10^-2 m^-1 at these velocities, so integrating the aerodynamic drag:

((e ^ (13.7 m * 0.013 m^-1)) - 1 ) / (0.078 s * 0.013 m^-1) = ~190 m/s muzzle
190 m/s / ((e ^ ( 0.013 m^-1)) ^ 13.7 m) = ~160 m/s impact

That's ~630 fps at the muzzle, ~530 fps at impact. For the initial velocity to have been in the region of 990 fps, it'd have to be covering the distance in closer to 50 ms.

[vng21092]

wow, you really did the math there eh? Thanks a lot. I based my calculations off the last shot. Subtracted what I thought was the sound of impact from what I thought was the sound of the muzzle, ended up with ~0.08 seconds, divided that by 45, and ended up with 562.5 FPS. I guess GGDT was pretty far off, that's kinda depressing . So now I'm curious, when air guns advertise a certain FPS, is that the FPS at the muzzle also?

@Gippeto, after downloading the app, I realized that was the exact one I had downloaded before that was giving me errors, I guess I'll give it another shot tomorrow.

[Ragnarok]

So now I'm curious, when air guns advertise a certain FPS, is that the FPS at the muzzle also?

"Muzzle velocities" are often measured a few metres from the muzzle*, as the escaping gasses can interfere with many forms of chronograph. For most projectiles though, the drag over that distance is insignificant compared to the shot-to-shot variation, so it doesn't really matter. BBs are fairly high drag compared to their weight though, so it is a little significant over that distance.

~~~~~

* Truth is though, advertised velocity figures are generally bollocks. They might have got the lightest pellet they could find out of one rifle that fast once, but it'd never be repeatable and that pellet would be stupidly light in any real circumstance.

Anything that advertises 1250 fps or something will almost undoubtedly give better real world results being fed a heavy pellet and perhaps even tuned down to get the velocity down below the transonic region. It'll be more accurate, have a better trajectory and be harder hitting.

The UK's energy limit on unlicensed airguns is almost a blessing on that front, because it kills "being the most powerful" dead as a gimmick, so rifles actually have to make themselves stand out in ways other than having a velocity too fast for their own good.
Something like my Air Arms TX200HC only does about 800 fps with its normal 8 grain pellets (which is enough for most of the things it's legal to shoot over here), but it'll put them through the same hole at 20 metres.

[vng21092]

So... I downloaded the app this morning and on the first shot, it gave me a reading of 635 FPS. Which I'd say is pretty O.K because my calculations through Audacity gave me 625 FPS. So I guess it works alright