calculations 2moa off, need advice

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by goose, Jun 12, 2010.

  1. goose

    goose Well-Known Member

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    I have just started working on long range shooting and have problems with my calculations. I can think of several reasons why they would be off and wonder what's the best order for the process of elimination. The problem has happened with two different rifles and I am usually off .5 to 2 MOA always high. Shooting from 400 to 500 yards. Today I was off about 2moa at 470 yards. I have used online calculators and charts in my reloading books, which both come out the same.

    I shoot on flat ground, the velocities I use to calculate are averaged over many rounds and conditions, some taken the same day.

    The optics are Leupold and Vortex, but I have never verified them. Should this be done at 100yds?

    I have no rangefinder, how reliable is Google earth? I'm using very easy to see landmarks, and think I'm getting good measurements on the program. I have also used measuring wheels that you walk behind, GPS and other things.

    I'm about 800' elevation shooting at around 70 degrees F, little to no wind.


    Possible problems I can think of:
    The bullet is traveling faster than I think
    The range is shorter than I think
    The adjustments on the optics are not accurate
    My shooting form

    Any more Ideas?
     
  2. Shane G.

    Shane G. Active Member

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    I've had the same happen my scope was not accurate supposedly 1/8"@100 but when rotating 1 full turn it was 10.5 higher,1 turn was 60 clicks so it figured out to .175"@100.

    I got a drop chart for my loads and was shooting at 620yrd. dialed up and was way high had to drop all the way back to where i had it marked for 550,next day after returning to zero it was on @100 go back to 620 and my dial up from the day before was not even close again.

    Seemed like even though it would return right back to zero somewhere on the way rotating up it was going off and not consistant.


    Also on your calculations your scope centerline to bore measurement will make a difference too.
     

  3. cowboy

    cowboy Well-Known Member

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    [QUOTE
    Possible problems I can think of:
    The bullet is traveling faster than I think
    The range is shorter than I think
    The adjustments on the optics are not accurate
    My shooting form

    Any more Ideas?[/QUOTE]

    You are on the right track in analyzing your problem. Could be any or combination of any of the above mentioned. Can you verify your velocity by borrowing another chronograph. My chronograph is consistently reading 30-35 fps too slow but is within the manufacturers plus or minus acceptable reading. Can you borrow a range finder to verify your velocity. I always check the adjustments on any scope - you would be surprised at the results. Assuming ammo comes from same lots of powder and primers as shot across chronograph - what do you get for readings of avg. velocity, extreme spread and standard deviation. How many rounds are you shooting to determine that you are .5 to 2 MOA high. Also - when you get .5 to 2 MOA high is this out of a cold clean barrel versus 2-3-4th shot or slighly fouled barrel?? Does your area get much barometric pressure differences - although at 4-5 hundred yards this shouldn't cause a great difference. How much difference does your rifle show between a cold or clean barrel versus having fired with a slightly fouled barrel. I have one rifle that after you clean it you better send about 4 rounds out the window before you do any analyzing or you could drive yourself nuts. Good news is that it seems like you are getting consistent results from actual to theoretical.

    Aside from all the above - the most consistent problem I have found from programs to actual is the bullet manufacturers published BC's. If your velocities are correct etc. try playing with the BC input to match your actual shooting data and see what you come up with. Do not assume that published BC's from bullet manufacturers are nuts on. A couple manufacturers are pretty good and some are very under on what they are published.
     
  4. LouBoyd

    LouBoyd Well-Known Member

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    deleted
     
    Last edited: Jun 13, 2010
  5. LouBoyd

    LouBoyd Well-Known Member

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    Goose, I agree with what others have posted. But I didn't see where anyone asked if you'd measured the distance between the centerline of your bore and the centerline of the scope accurately and entered that distance into your ballistics program. All ballistics programs I've seen have a place for that and it's important. Having that entry wrong by about two inches would account for the 2 MOA error with a 100 yard zero when shooting at 400 yards and beyond. If you set it better than about 0.1" the error won't be noticeable .

    There are other possibilities which could be causing a 2 MOA error but that one is easily missed by new users of ballistics programs.. Been there, done that. Wrong muzzle velocity, air density, and bullet BC are also possible errors but less likely in my opinion. Two MOA is a lot. The scope or rifle having a problem is very unlikely.

    If you're serious about ballistic calculations do get a range finder and a chronograph. For hundreds of years shooters were reasonably successful with neither. That came from lots of practice, some in estimating distance and holdover but mostly from learning to stalk close the their targets.

    Google Earth can be used for distance measurements with an accurate of a couple of yards at reasonable shooting distanced (further than most range finders) but only IF you can identify objects on the ground. The photos can be ten years old or more. Google earth has a built in tool for measuring straight lines between points. I'm not sure if if gives the slope distance or the map projected distance but it does not give the slope angle. One possible advantage over a range finder is it can give the distance even though a hill (for those high trajectory shots).
     
    Last edited: Jun 13, 2010
  6. elkaholic

    elkaholic Well-Known Member

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    Goose...I agree with other posts. What bullet were you using? Also, what was your load and barrel length......Rich
     
  7. bigngreen

    bigngreen Well-Known Member

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    One of the most common problems with calculating trajectories is how you deal with elevation and Barometric pressure, if your using Station pressure you won't input an elevation but it you use corrected pressure like you get on the weather channel then you need to use an elevation with it.
    Some one mentioned scope high, that will really through of the calculations in the ranges your having problems.
    Going from shoot at a bench to of a bipod will change things, there's a bunch of stuff that will do this, you just need to work through each possibility.
    Sometimes posting all the data your inputting into the ballistics program will help others find a problem in how your using the program.
     
  8. LouBoyd

    LouBoyd Well-Known Member

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    Applied Data Science makes a little USB device which outputs air density direclty . It's $159 and outputs a bunch of parameters in in ASCII. It calculates air density internally from three sensors which measure absolute air pressure, air temperature, and humidity. It can be used with just about any modern computer operating system and comes with Windows sofware. XP or newer)
    Applied Data Sciences / AirTool II

    There are also mechanical air density gages. They're commonly used for tuning racing engines. They're a little over $100 and don't need power or a computer. They're somwhat delicate, like a mechanical watch. It works by actually weighing a small volume of air with a balance and a weak spring.
    Longacre Racing Products 50888 - Longacre Air Density Gauges - Overview - SummitRacing.com

    I have both I believe the two methods are about equally accurate (under 5% error). Neither require additional information like elevation or local barometric pressure.
     
  9. goose

    goose Well-Known Member

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    shooting a 270 win, 22" bbl 130gr sierra sbt 54gr of rl 17. I have vel. from 4 or 5 groups of 5 shots. the groups have sd of 12 to 25 and average from 2915fps to 2980fps from the slowest group to the fastest. The chrony is about 20-25' from the muzzle. The shots at distance were after 7 or 8 shots post cleaning. Sight height is about 1.6 to 1.7 above the bore.
     
  10. LouBoyd

    LouBoyd Well-Known Member

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    I presume that's centerline of bore to centerline of the scope. Is that value entered in the ballistics program you're using? It should say so in the program's output. What ballistics program are you using? If the program gives a trajectory table with header information could you post it ?
     
    Last edited: Jun 13, 2010
  11. LouBoyd

    LouBoyd Well-Known Member

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    I ran some quickload and quicktarget simulations to see if I could figure out where the 2MOA difference from a 100 yard zero to 500 yard point of impact is is coming from.

    Quickloads calculation of muzzle velocity from your load info gives 3026 vs your measured 2980. That's pretty close and only accounts for 0.3 moa.

    Had you used zero for the scope height instead of 1.7" in the ballistic program that would account for 1.3 MOA at 500 yards.

    Had you used the database information on the Sierra 130 Pro Hunter bullet (.370 G1 BC instead of the Sierra Gameking (.438 g1 BC) that could account for only .53 moa

    I'd still bet on the problem being a measurement or data entry error, not anything wrong with the rifle, scope, bullets, or loads.
     
  12. goose

    goose Well-Known Member

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    I've been using Hornady's online ballistic resource. I've also checked with some printed tables.

    The first thing I'm planning to do is take some shots at 100yds off the bench. I haven't changed anything since getting it dialed in at 470. How high should I be at 100?
     
  13. bigngreen

    bigngreen Well-Known Member

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    Are your your velocities corrected for the distance from your muzzle to the chrony? As far as your shooting it will be important.

    LouBoyd that some interesting tools you posted the links for, I kinda scanned them but will look them over good.
    I use Loadbase 3.0 for my ballistics program and it has a analyzer that I can input Baro, temp and humidity in and it will give me the DA to use if I want to go that route. I normally just input it directly into the tables. Is there and advantage to using DA over other methods or is it another tool.
     
  14. LouBoyd

    LouBoyd Well-Known Member

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    Quicktarget shows the velocity loss at 20 feet is 45 feet/second. That would be 3025 at the muzzle if it' 2980 at 20 feet. Quickload predicted 3026 fps MV when I ran it independently. That's too close to be believable ( typically Quickload is within 50 FPS at 3000 fps.