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Posted: 6/8/2010 4:00:17 PM EDT
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Can someone tell me if there is a way to find muzzle velocity without using a chronograph?
(Can't buy a chronograph, poor college student  )
Here are my specs if there is a way. rifle Weatherby Vanguard 308 24" barrel 1 in 12 twist bullet Nosler CC - 168 gr Tac - 44.4 gr Win. brass CCI primer |
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Along the pure physics line - you can shoot the round straight up and then time how long it takes to come back down. Proper application of math will give you the muzzle velocity. It's the (really) old school approach and it requires shooting from a platform in the middle of a suitble lake or pond on a near windless day so you can spot the splash.
It's a LOT easier with a chronograph. |
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Traditional way was to fire a bullet into a 'pendulum' of known weight, and recording how far it moves. Then calculate the force applied, and the velocity.
ETA wiki link: http://en.wikipedia.org/wiki/Ballistic_pendulum Or, check out this computer based sound card chronograph: http://arxiv.org/ftp/physics/papers/0601/0601102.pdf |
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When I was in college as a electronics major decades ago, I built a chronograph from off the shelf stuff for about $20. used a 10 Mhz crystal oscillator into decade counters and LED displays. Didn't have "skyscreens" back then, so I just shot through magnet wire grids spaced 12" apart, using a XOR gate for start / stop. Not very sophisticated, but it did work.
Still have that somewhere in the basement..... |
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Since you are a poor college student, you should build your own bullet chronograph. It should be cheap and you can give your brain a good workout building it. Watch this youtube video and in the video you will see some spinning targets. That is a chronograph.
jonblack |
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Quoted:
Along the pure physics line - you can shoot the round straight up and then time how long it takes to come back down. Proper application of math will give you the muzzle velocity. It's the (really) old school approach and it requires shooting from a platform in the middle of a suitble lake or pond on a near windless day so you can spot the splash. It's a LOT easier with a chronograph. Those would be some hard numbers to crunch, especially since the bullet will not come down as fast as it went up. Any error in measurement would throw the calculated result off by an unacceptable margin. jonblack |
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Quoted:
Since you are a poor college student, you should build your own bullet chronograph. It should be cheap and you can give your brain a good workout building it. Watch this youtube video and in the video you will see some spinning targets. That is a chronograph. jonblack In case you want to fast forward to the exact spot: its at 6:47. jonblack |
| If you know the BC of the bullet you are using, you could shoot it at two known ranges (100 & 200 yds for example) and measure the drop. From there, you can use theJMB Ballistic Calculator and figure out what velocity would give those results. |
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Before electronic chronys, velocity was measured with an inertia system. Using a known mass of the bullet, and the amount of deflection of a known-mass target, one applies basic math and physics. The problem with this is that it included error-prone transcription of data and error-prone manual math.
Save up some coin by skipping a couple of lattés a week, and buy a Shooting Chrony Beta Master for around $125. You want the "Master" version because the only things downrange are the skyscreens and their holder frame-because eventually that downrange part WILL get shot. |
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Quoted:
They come down base first as well....I never claimed it was even remotely easy. Quoted:
Along the pure physics line - you can shoot the round straight up and then time how long it takes to come back down. Proper application of math will give you the muzzle velocity. It's the (really) old school approach and it requires shooting from a platform in the middle of a suitble lake or pond on a near windless day so you can spot the splash. It's a LOT easier with a chronograph. Those would be some hard numbers to crunch, especially since the bullet will not come down as fast as it went up. Any error in measurement would throw the calculated result off by an unacceptable margin. jonblack 
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Quoted:
Back when chronos were expensive and used what amounted to single use shoot through screens, this is roughly similar to what I did.
If you know the BC of the bullet you are using, you could shoot it at two known ranges (100 & 200 yds for example) and measure the drop. From there, you can use theJMB Ballistic Calculator and figure out what velocity would give those results. Most bullet manufacturers publish the BC's for their bullets and Sierra tends to publish BC's at different velocity ranges. In practice, it takes a fairly large change in BC to have a significant impact on trajectory until you start getting out past 400 yards. In other words a bullet with a BC of .280 won't shoot a lot flatter than a bullet with a bc of .240 all other things being equal, but you'll see the difference with a bullet with a BC of .400. What it all means is that if you know the bc and have a loaidng manual with ballistics data, (Hornady, Sierra, etc) you can compare the ballistics data in the manual for your bullet, or a very similar bullet with what you actually get on the range. So in effect, you'd refer to the manual and based on barrel length, powder charge, etc estimate what you expect for velocity. (The closer your rifle matches the test rifle and/or barrel the better, and the distance between the center of the bore and line of sight is important as well) Then you go out and shoot it over as long a range as you have available and compare the results on the targets at various ranges with the expected results for the weapon zeroed at the specificed distance. You will usually find ballistics data in the manual that match your observed results within a 100-200 fps one direction or the other from your estimated velocity, or you will find two that seem to bracket your observed results, giving you an extrapolated velocity somewhere in between. Today, you have the added option of ballistics software. I used to have Sierra's Ballistic program (technically I still do but no longer have a computer that will take a 3.5" floppy - and that was an upgrade from the original version on a 5.25' floppy) and it was very capable stuff and I am sure the current version still is, but it was expensive and probably still is. There are some free options available. The JMB ballistic calulator above is very capable and easy to use. Lee offers a free ballistic calulator you cna load on a lap top and is then not internet dependent. The interface is a little odd, but easy to use once you figure it out and has a very robust data base of bullet data for various manufacturers. In either case, the process is abotu the same - shoot the rifle and see what you have, then run a table on the estimated velocity and then tweak the velocity and re-run it until you get results that are close to what you observed. That then gives you a very close estimate of your muzzle velocitry based on real world ballistics. What you lose in comparison to a chronograph is the ability to measure individual velcities and then analyze them for mean, range and standard deviation. Those measures of central rendency tell you a lot about the limitations of the load (the expected variation in drop and windage you can expect at longer ranges) and they also tell you a lot about the quality of the load, and it may drive soe decisioin making on load selection. For example a load with a SD of 45 fps may be exceptionally accruate at 100 yards where the differences in velocity do not begine to show up, but at 400-600 yards, the expected extreme spread of about 135 fps will start to show up in significantly more vertical stringing even on a dead calm day. In that regard, you may elect to stay with a load that has slightly larger groups at 100 yards but has an SD of only 10 fps as the long range accuracy will be better, or at least more predictable. In the long run, the benefits of a chronograph are immense and the cost today is very small compared to what it was in the past and it is well worth the investment. it is also worth the investment in terms of load development and in fewer rounds wasted trying to plot the trajectory and reverse engineer the velocity. A Shooty Chrony for example is very compact and portable and even the base model offers more features than a state of the art chronograph 20 years ago. And they sell for only $90. Mount it on a cheap camera tripod and you are good to go for under $125. |
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