How to check a Chronograph?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Wile E Coyote, Nov 8, 2011.

  1. Wile E Coyote

    Wile E Coyote Well-Known Member

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    Earlier today I ran across a thread here on LRH about Chronographs and I'm hoping someone could help me sort out a problem I'm having with my Beta Master Shooting Chrony, a rifle and load combo.

    My question is how to prove the calibration of the chronograph?

    I recently did shoot across 2 other chronographs on 2 different occasions. First time, all were within a few FPS of each other. The second time, a few weeks later, there was a very wide difference – off by hundreds of FPS. My Chronograph was the slowest.

    A Tech at Shooting Chrony suggested that I could get some factory ammo with a "known" velocity, shoot it and see the results. Seemed reasonable, so I did just that with the rifle in question (a 7 mag) and several others in different calibers. While each was somewhat consistent (factory ammo), they were not what was advertised and off by more than I would have expected. Some were faster while others were slower. My conclusion here is nothing proven or dis-proven with this test.

    Could anyone help?
     
  2. Tumbleweed

    Tumbleweed Well-Known Member

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    For the tech at Shooting Chrony to tell you to check your chrony accuracy by running factory ammo with a "known velocity" through it is crazy. There are simply WAY too many variables for factory ammo to run what the factory rates it at-such as minor little things like barrel length. That "test" will not do you any good. I too have been through the headache of not knowing which chrono to trust. What I did was perform a drop verification to 1000 yards and recorded drop information at a few distances in between 100 and 1000. I then compared that info to JBM's drop data with Bryan Litz's ballistic coefficient for my bullet. I started with inputting the velocity I got from my chrono and then made input adjustments to the program until my actual drop and the predicted drop matched. This worked great and gave a LOT of peace of mind.
     
  3. Michael Courtney

    Michael Courtney Silver Member

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    You need an independent way to know the bullet velocity. We usually use multiple chronographs in most situations. I also published an acoustic method a few years ago that will work:

    http://http://arxiv.org/ftp/arxiv/papers/0812/0812.4752.pdf

    I've also got a few loads that I've used for many years that have a well known velocity and standard deviation in a given rifle. I'd steer away from factory ammo unless its velocity has been verified in the same rifle, because I've seen too many loads 200-300 fps difference from the factory claim.
     
  4. TexasHunter

    TexasHunter <strong>SPONSOR</strong>

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    Hey Wile,

    During the load dev and testing of the one mile rifle we built for NTRP, we used a beta and a CED to test loads. With the exact same load, the Beta gave us around 3175 fps and the CED gave us around 3000. Both units were set up at the same bench at the exact same distance. All reloading components were the same as was the load itself. We were developing loads for the 300 RUM using the Hornady 225 grain match. We really needed around 3100 fps to get to the mile with the 225. Needless to say, we were scratching our heads. I called Hornady to confirm the BC of the 225 and they told me .670 G1. My client, Josh Ruby of NTRP had used this bullet extensively in a 300 WinMag and believed it to have a BC more close to .690.

    So the following weekend we went to his 1780 yard range to test rifle and load. We were getting an SD of 5 from our load and with the Tubb2K brake, we were able to tune to a 100 yard 3 shot group of .4" outside to outside. We used our Kestrals to measure the DA, made the calculations with Ballistic FTE on our iPhones using direct DA input and sent the first round. Slightly right of the 20"x20" steel but with perfect elevation.

    The answer is that we used the 3175 fps number from the Chrony and it was right. The CED was way off. This really is the gold standard of determining whether or not a chrono is correct in my opinion. BTW, I purchased a new Oehler 35 three screen that same week because I got tired of the uncertainty. Although i have to say the the cheap little Chrony has served me well for almost 20 years.

    In most cases I can remember when my calculated dope was off at 800-1000 yards, it could be tracked to the bullet's BC being substantially less than advertised. I shoot a fair bit of long range tactical events and all the shooters compare and share ballistics. So when a dozen guys tell you a particular bullet is different than published and you're seeing the same thing, you gotta believe it. You also have to be anal about tracking all the relavent data. If you sight in to the north or south and you're shooting east or west (or any variation on this theme), you need to turn on the Coriolis function in your ballistic calculator. At 1000 yards with a 260 Rem, there is .3 MOA of Coriolis shooting east or west. Without this, you could assume your measured speed or your BC was wrong.

    Hope this helps.

    [ame=http://youtu.be/NQqjhCCh60Y]Thoroughbred Rifles One Mile Rifle in .300RUM - YouTube[/ame]

    Dale
     
  5. Wile E Coyote

    Wile E Coyote Well-Known Member

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    Before I get lost in everyone's ideas and comments, let me say thanks for all of your input. Thank you.

    Tumbleweed, like you said, I too was not thrilled to hear the tech's response either because he didn't sound too sure of himself. He really wanted me to send it back for testing in their shop. but gave the suggested method a try anyway with 22's, a .223 and a 60+year old crossman co2 pistol. The only one that was absolutely consistant was the old pellet gun.

    I used the ballistic calculator here on LRH to match some drop data I shot a couple months ago. That comparison is what really confused me. I should have mentioned this earlier but wasn't thinking. more in a moment.

    Michael, I downloaded and read your paper (quickly) on acoustic measurement. I understand the concept but I will have to reread this later when I can wrap my head around this a bit tighter. thanks.

    Dale, shooting the data seems to be the way to go just as you did and also as tumbleweed mentioned in his comment. I'm beginning to see where I may have gotten ahead of myself by relying on just the chronograph.

    Having swallowed the BOTW and Huskemaw hemlock a couple years ago, I started to build a rifle and a load similar to what they used on their videos and tv when they were sponsored by Gunwerks.

    Here is a summary what I've done to date; This summer, I completed the rifle project. A Remington 700 in 7mm Rem Mag. Rebarreled by Hart with a 26" 1/9 twist, action squared & trued and some other goodies too. I'll take some pics and post soon.

    After 60 rounds of break-in with Hornady factory ammo I reloaded the brass and quickly found a really good and consistant group at 72.3 grains of powder with 0.010 off the rifling. absolutely no pressure signs.

    The cartridge load: Hornady brass, Retumbo, CCI-250, and Berger 168 Hunting VLD's.

    Several groups of 5 at less than 1/2 MOA at 100yd. My Shooting chrony data was just as good, or at least it looks that way. Average velocitys of 3031 for 10 rounds and another day 3035 for another ten rounds. The ES was 10 and 13 while the SD's were single digit. I really thought I stepped in it with these numbers.

    In late August I was out to the farm to hunt woodchucks and shoot out to about 500yd. We used the opportuinity to rezero at exactly 200yd and begin a drop chart. Setting targets at 300, 400 and 495 measured with a laser, I essentially just guessed at how many clicks to add to the dope to get the elevation and adjusted from there. Once the range/click values were determined, I shot a couple rounds at each again to prove it. Recorded not only range but angle, temp, station pressure, and humidity. Elevation is 600 to 700 ft. That afternoon the woodchucks were none too happy with me and that rifle.

    In September we again went to the farm and shot the same targets and interpolated several inbetween targets to prove the earlier data. The 400yd and 495 yd groups were all considerably less than 1/2 minute, in fact, I shot a hole through the 10x10x3/4 steel at 400 with 5 rounds. Recorded the new data and again sucessfully became a woodchucks worst nightmare.

    I took all this data and plugged it into the Ballistic Calculator at LRH and found the data did not match. not even close. The data input was double checked and found OK. I couldn't make MY data match the Ballistic Calc Data until I had advanced the muzzle velocity up to 3255. Then it works almost perfectly. This is my first experience with a 7 mag and 3255 sounds way too fast. I've yet to find a manual that moves a bullet at 168 gn this fast out of a 7 mag. And here is where I don't know if I should trust the chronograph or the program.

    Either way, the rifle shoots beyond my expectations. I just can not quantify the performance completely. If anything, I just want to get the ballistic turret for the Huskemaw scope.

    Thanks again

    Pete
     
  6. Tumbleweed

    Tumbleweed Well-Known Member

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    Pete, still sounds like you've got an input issue with your program. There's no way you're running 3255fps. Your chronographed velocity of 3035 is very realistic and is probably right. Also, you will want to shoot a lot farther than 500 yards to document your true drop on paper (if that's a possibiliy). You will get a lot clearer picture of what your bullet is doing.
     
  7. Wile E Coyote

    Wile E Coyote Well-Known Member

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    I have arrived at the same conclusion of data input plus another possibility; data recording at the time collected resulting in a defective input. I'm usually kinda thorough about this sort of thing. Sometimes to the point of being annoying to some.

    Next week, I'm may have another chance to shoot at the farm. I'll re-record all the data and try again out to 500. That is the longest safe shot I have at this location. At another location there is a 700 to 900 yd shot but nothing shorter. There is also the possibility to take a 1700+yd shot at a 21deg down angle back where I hunt but I won't shoot this one until I'm absolutly sure of everything.

    thanks again
     
  8. TexasHunter

    TexasHunter <strong>SPONSOR</strong>

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    Wile,

    Don't know what your calcs were but here's what my calculator gave me.

    Berger 7mm / 168 / G7 BC = .316 / 3030 fps / sight height 2"

    DA=1000, 300yds=2.9moa, 400yds=5.0moa, 500yds=7.4moa, coriolis off

    DA=2000, 300yds=2.9moa, 400yds=5.0moa, 500yds=7.3moa, coriolis off

    DA=3000, 300yds=2.8moa, 400yds=4.9moa, 500yds=7.3moa, coriolis off

    DA=4000, 300yds=2.8moa, 400yds=4.9moa, 500yds=7.2moa, coriolis off

    DA=5000, 300yds=2.8moa, 400yds=4.9moa, 500yds=7.2moa, coriolis off

    DA=5000, 300yds=2.8moa, 400yds=4.9moa, 500yds=7.1moa, coriolis on @ 90º

    In general, Berger BCs seem to be the most accurate of the different makers. Using 3255 fps drops the 500 yd dope to 5.9 moa with coriolis on at 5000 DA. I stopped at 5000' DA but that doesn't mean you weren't shooting in a much higher DA. I shot a match in OK back in Sept where the actual altitude was 1200'. When we started at 8am the actual DA was 2100' and by the time we finished at 4pm, the DA was almost 5000'. We were shooting east on every stage and back at the range HQ after the match several guys were complaining about their dope being too high out at 1000 yds toward the end of the match. I asked if they input DA and they said "sure we did, this morning". When they re-measured and re-input while we stood there, they were pissed that they hadn't considered the massive change as the day warmed up. There is also the possibility that different bore / groove configurations have varying effects on the actual BC but it would have to be huge to change the 500 yd dope by 1.2 moa so not a very likely culprit. Head and tail winds can also effect elevation to some degree but again not this much this close.

    Bottom line is that I personally fought this issue for years and until my wife gave me a Kestral 4500 last Christmas and I was able to measure and input DA directly, I never had confidence in my calculations. I would just keep modifying the inputs to match my 500 actuals but they were still off at 1000 yds. Keep looking, you will figure it out.
     
  9. TexasHunter

    TexasHunter <strong>SPONSOR</strong>

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    One more question - did you zero at 100 and input 100 as your zero range? My calcs above used a zero at 100.
     
  10. Wile E Coyote

    Wile E Coyote Well-Known Member

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    Thanks Dale, The original zero was 100 at the range for break-in and shooting the first few groups on the bench. On the first trip to the field with this rifle, that zero was the first thing adjusted; to a measured 200. The turret was adjusted to 200 and that is where it stays.

    As for the actual distances, we have a couple of markers ( Paint marks on exposed bedrock in the field) we actually measured with a tape years ago. My 300 and 495 distances are also marked similarly and all have been rechecked whenever a new piece of technology comes along. For instance, this summer I replaced my old rangerfinder with a Leica CRF1600. A 8" square piece of aluminum foil on a target board (and a plain paper target too) returned exactly
    what we measured years ago.

    I'll check the numbers you calculated against my notes when I get home tonight to see if there is any similarity. Next week I'm going to shoot again. I'll start with a clean piece of paper and record everything I can with the rangefinder and Kestral 3500. Then we'll try again to use the ballistic program to "back into" the velocity along with checking and confirming the drop table I have already made.

    Pete