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problem cronographing loads

Discussion in 'Uncategorized Threads' started by lewddt10, Jul 22, 2007.

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  1. lewddt10

    lewddt10 TS Member

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    shot Winchester super target 3 dram, 1 1/8oz. 71/2 shot, on box says 1200 fsp. on box. shot 1310 fps. and shot Remington nitro 27's, 1 1/8 oz. 7 1/2, on box says 1235 fps. and shot at 1330 fps. both these loads shot at almost a hundred feet per second faster than what it says on the box. Shot 5 ft. from cronograph and 6-8 inches high which is what it said in the cronograph instructions. Shot from a Ljutic pro 3, 34" barrel, ported with a light-full choke. Does anyone have an idea why these are shooting so much faster than it says on the box? Any help would be greatly appreciated.
    Thank you, Jeff
     
  2. rennerize

    rennerize Active Member

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    I am told that the published fps are for 30" barrels and you can add about 20 fps for each additional inch of barrel length.
    Don
     
  3. Straight99

    Straight99 Member

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    I have to go about 10 ft back and then I get a normal reading. Cloudy days seem to work better.
     
  4. C H S

    C H S TS Member

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    The consumer style chronos always read higher than the factory says because they clock only the first pellet through the field. Factory plate chronos clock the whole shot cloud behind the lead pellet and it's behind because it's moving slower. Dick Furrier at Alliant Powder told me this and also said that there is no real good way to determine how much faster the hobby chrono reads than the factory equipment.

    Andy
     
  5. BDodd

    BDodd TS Member

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    Barrel length will increase or decrease readings. Temperature will affect readings. The tighter the choke, the faster the reading on your chronograph. Oehler recommends using either cylinder bore barrels or at least the most open choke you have available. When I was working more with the Chrono, I found one set of my O/U barrels, 28", skeet and skeet chokes, would provide almost exactly what boxes said factory shells should be. I then used that barrel, gun, and choke for all my tests of reloads.

    The best fix for those beginning with chronograph work is to test factory loads you want to replicate and don't worry much about the fps reading. Then build your reloads to match those readings using the same barrel, gun, and choke. If you want to match those Winchester or Nitro loads, Jeff, just make a couple of tests of 10 shots each with the factory loads and then try your reloads on the same day, same temperature, same position in the sun or shade, same barrel, same gun, and same choke and that will get you in the ball park. Need more speed to match the factory? Follow a recipe that will get more speed. Too Fast? or you want to come in between two factory loads? Adjust your recipe within published data and there you go....breakemall....Bob Dodd
     
  6. zzt

    zzt Well-Known Member

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    Jeff, my first chrono said my reloads, which were supposed to be 1150fps were 1220fps. You should have se teh looks on the faces of those watching me as they quickly backpedaled to what they thought was a safe distance. I'm sure they thought I was going to blow myself to bits. Since I had used that load without incident for several years, I knew it was a bad chrono and not the load.

    I returned that chrono and received a replacement. It reads 15fps different than an identical model at the club. I've learned that you have to accept the velocity readings as relative. Much as Bob Dodd says.

    The default for "factory" measurements is 59 deg at sea level. I forget what the humidity reading is supposed to be, but it isn't that important. Shooting through a full choke, as I do, adds 30fps to your reading. Shooting at 90 degrees adds another 25 to 30fps, depending on powder. So does shooting close to the chrono.

    I've found that no closer than 6' from a light operated chrono is more consistent. If you get a lot of fliers, more farther back. Since you cannot measure the absolute velocity of a load, only the "relative velocity", what does it matter what the chrono says. As suggested above, more back, shoot the factory load you want to match as a control, then shoot your reloads for comparison. If you come close and get SDs in the single digits or low teen, you are good to go.
     
  7. lewddt10

    lewddt10 TS Member

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    thanks everybody for the help. B Dodd- did not want to match the Winchester loads but wanted to match Nitro loads. Loaded STS hulls, Winchester 209 primer, Winchester WAA12 wad, 22.5 grains of PB powder. Shot at 1325, average of 10 shells. Thanks, Jeff
     
  8. BDodd

    BDodd TS Member

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    Jeff, well there ya go, sounds like you're already matching those nitros when compared to the real thing - all assuming the same gun, barrel, choke, temps, set up were used for both. I'd trust it if that recipe is published but I'd repeat the test a couple of times to ensure no flaws anywhere....Bob Dodd
     
  9. Quack Shot

    Quack Shot Active Member

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    It's not uncommon with tight chokes and high temperatures to get readings substantially higher than what you'd expect. I'd back up a few more feet, use the diffusers over the screens, and get the temps closer to "room" temp.

    The best way to get accurate velocity readings is to use induction screens. They cost more than I'm willing to spend for them. At normal temps (70-75 degrees), most of my readings are about 25 fps higher than expected, using a more open choke.
     
  10. halfmile

    halfmile Well-Known Member

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    We ususally subtract 30 FPS from the Chronograph readings and come very close to what we expect as reality.

    5 shot averages, and throw out any aberrant readings. Use the screens if there is even a hint of sunlight.

    My friend would Chrono a 5 shot batch of factory Federal papers, and then adjust his PB loads to match. A new lot number meant a new Chrony session. It must have worked, I saw him shoot 475 straight once, counting the event targets and the shootoff.

    HM
     
  11. Neil Winston

    Neil Winston Well-Known Member

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    But Halfmile has brought up an important point. What to do about "aberrant readings?"

    If you are testing for speed, then throwing them out is fine. If you are testing for consistency, it's no good, and for that you will need two chronographs mounted on the same rod about 2 feet apart.

    That's when you really discover the problem with chronographing with full chokes. If you use cylinder, both readings will be the same, all the time, really. With full, they will be as much as 30 ft/sec apart, on a random basis. So you can't test consistency with full chokes; the readings themselves vary as much any "real" differences you are looking for.

    Neil
     
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