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Old August 9, 2005, 09:09 AM   #1
ethernectar
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Bullet Trajectory different than computer model

I shot my first tactical long-range match sunday. My POI was consistently higher than the computer ballistics program predicted. Now, the temp was significantly higher than I had calculated for (20 degrees) but when I got back home I recalculated for the temp (100ºƒ) and the POI was still higher. What input data could I have input wrong to result in the data being that far off. At 1000 yards I was something like 20+ clicks high.

The rifle is a JP Custom Sendero in 300 Win Mag. I'm still finding the 'sweet' load for it, but it'll shoot a 1/4" group at 100y.

m
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Old August 9, 2005, 09:16 AM   #2
Jim Watson
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Hanged if I know.
Most published BCs are optimistic and anything I have ever shot hit lower than predicted by chart or computer. 20 clicks on your scope is what, 5 MOA? That seems like a lot. What was your starting point, 100 yards or farther out?
I think the thing to do is to shoot at as many different measured ranges as possible and make out your own table of fire to go by instead of a computer printout.
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Old August 9, 2005, 09:40 AM   #3
Zak Smith
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Hi. My friends and I do a lot of long range "field" shooting out here, at different elevations. We've found that using a program like Sierra or Exbal, the real trajectory follows the computed trajectory pretty well, within 1/2 MOA anyway.

The important parameters to get right include: BC value(s), sight over bore, muzzle velocity, environmental conditions (altitude, pressure, humidity, temp; and/or density altitude), and of course a solid zero distance.
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Old August 9, 2005, 09:45 AM   #4
Leftoverdj
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Scope?

If one click is .30 or .35 MOA rather than .25 MOA, that would explain your results.

I'm a "shoot it and see" type myself.
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Old August 9, 2005, 11:07 AM   #5
Mal H
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I'm with Leftoverdj on this. How did you determine what the initial scope setting should be?

And a far fetched, but possible solution to the problem - are you sure you're shooting at 1000 yards? Is it at a measured rifle range?

Have you chronographed the round to see what the actual bullet speed is?
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Old August 9, 2005, 11:11 AM   #6
Zak Smith
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I'll add that I was able to use the computed elevation for 1 mile with my 50BMG (19.1 mils), and it was "on" within the limit of vertical dispersion with the surplus bullets and powder.
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Old August 9, 2005, 12:00 PM   #7
moredes
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I think most of the experienced long-range, multi-caliber shooters (I'm not one, I just read their advice) will tell you that there's "no one size fits all" ballistic software that'll project accurate results across all calibers/loads.

Be satisfied that you'll find paper at the longer distances with the computer data. That's really all it's for; and in my experience, (w/ a single caliber, .308) I've not been able to find a program that can consistently project 168's, 175's, and 178's in 100yd increments out to 1000yd with corresponding computer-projected accuracy from weight to weight. I'm on paper, for sure; but the rest is 'field work'.

The same guy traveling across the country shooting at the same elevation and temp can get different results just because of light and humidity. Ballistic software hardly allows for considering those conditions.

You said you were 5MOA high at 1000yd. Project that in your software; what do the 500-900 yard projections look like; that is to say, do they tally with your actual results, or are they "about" as far off as your field work?
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Old August 9, 2005, 12:01 PM   #8
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I find ballistics calculators to be of limited use--decent enough to get you on paper, but don't expect any more than that. If you had the correct BC (hooot!), plus current temp, humidity and altitude, they're a little better than the what I can guestimate off the top of my head.

Still, 5MoA is a lot of error. Check the sight height and MV. If you poke around, there are a couple websites that have BCs that are calculated rather than pulled right from the vendor.

Ty
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Old August 9, 2005, 01:11 PM   #9
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When I began long-range shooting, I dutifully recorded the published BCs of the bullets I was using. Then I went to the range and found they were different.
I use a 155 grain bullet that is supposed to have a BC of .450. That's close enough to get me on the target but far enough off to make me fiddle with the numbers. According to my notes, my come-up for 300 yards is 4 MOA, for 600 it's 14 MOA, for 800 it's 22.5 MOA, for 900 it's 27.5 MOA and for 1000 it's 36 MOA. I plug different BCs into Pejsa's program until I can come close to all those values and find the real BC is closer to .393.
I have spoken to a few representatives of bullet manufacturing companies and have asked them how the derive their BCs. Invariably, they tell me the data comes from their "Ballistics Lab." as if such a lofty pronouncement will stifle any other questions. I have yet to see data from Sierra, Nosler or any other company that indicates they actually measure the trajectory of their bullets. In the absence of any such data I have to assume their values are established as much by their marketing analysts as their ballisticians.
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Old August 9, 2005, 01:20 PM   #10
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BC measurements are normally done by measuring the time it takes the bullet to pass through screens from near-muzzle to the maximum distance, using the current environmental conditions, and comparing the time to the base bullet for the drag model used (e.g. G1, etc)
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Old August 9, 2005, 11:40 PM   #11
ethernectar
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This is a measured range (Sacramento county range)

Scope is a Leupold Mk4 6.5-20 from Premiere Reticle. 1/4 minute clicks. Definitely not a budget-minded optic.

The match shoots at 200, 300, 500, 600, 800, 900 and 1000 yards. Was high from the beginning. At 800 I turned 7 fewer clicks than the program recommended, which got my elevation closer to right. At 900 I turned a couple clicks less than prescribed and likewise at 1000.

Sight height was eyeballed with a micrometer at 1.75, not sure how precise that measurement needs to be.

Chrono'd a small batch the previous day, those chronos averaged out to 3006 FPS. But it was cooler by some 20 degrees+. However, even with putting in the temp to the software at 105, it didn't make up the difference.

Load Details:

Virgin 300 Win Mag Brass from Winchester
Winchester Large Rifle Primer
77gn Reloader 25 Powder
Sierra 190 Matchking HPBT
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Old August 9, 2005, 11:48 PM   #12
Zak Smith
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Sight over bore distance only need to be within about 0.1 or 0.2" - you're probably OK.

Was the gun heating up? The powder temperature can have an effect depending on the composition of the powder used.
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Old August 10, 2005, 12:18 AM   #13
Mal H
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Something really strange is going on ethernectar.

You say that, in essence, you zeroed the rifle at the 800 yard point. Yet when shooting at 1000 yards with about 4 clicks added elevation, the rifle is shooting 20+ clicks high which is 5 MOA. At 1000 yards 5 MOA is about 52 inches high.

I plugged in your data into the Sierra Infinity calculator using the same bullet and MV. When zeroed at 800 yds, the drop should be around 82 inches at 1000 yds. Allowing for the 4 clicks or 1 MOA you added from 800 to 1000 yds, that means the POI should have been around 72 inches low. You are actually getting about 134 inches elevation when going from 800 yds (zero) to 1000 yards. ???

I don't supose someone else was shooting at your target by mistake?

Ooh, ooh! I just had a thought! (No comments on that, please. )
How high off the ground is the target at 1000 yards? Would 72 inches low mean the bullet is hitting the ground in front of the target? IOW, could you be getting a bouncer?
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Old August 10, 2005, 10:52 AM   #14
ethernectar
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Not quite, didn't zero it at 800, just reduced the number of clicks that was recommended. The rifle was zero'd at 200 yards. The first few firing points the rifle was consistently high. So at 800 instead of turning the prescribed clicks, I clicked 7 fewer (as recommended by an experienced shooter next to me). That adjustment had my group basically centered on the target. It seems like my trajectory is much flatter than what the software predicts. What would cause that.

Now I understand that a hot barrel/hot conditions will mean you need to take a couple clicks off to adjust for the heat. But I'm thinking there's something else going on...

m
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Old August 10, 2005, 03:51 PM   #15
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Ok, I'm still confused - not that uncommon.

As I see it, only 3 things can cause the phenomenon you are experiencing if all other factors are as you determine them to be (scope, distance, etc.):
1) You are shooting in a near vacuum,
2) You have changed the gravitational constant,
3) Your muzzle speed isn't what you think it is.

I believe only #3 is one you have some control over.

You say "That adjustment [at 800 yds] had my group basically centered on the target." The bullet will be falling rapidly from 800 to 1000 yds. You added around 1 MOA adjustment and the bullet is impacting very high on the target at 1000 yds. IOW, the bullet is actually rising from 800 to 1000 yds, because 1 MOA is not enough to account for the fall that is expected between those two distances. I'm not sure what I'm missing, but it's something, and it's important.
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Old August 10, 2005, 11:36 PM   #16
ethernectar
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Will have to get my buddy with the chrono and go out again.

m
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Old August 11, 2005, 05:14 AM   #17
WESHOOT2
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what caliber computer?

Remove program.
Place media on floor.
Do happydance on top.

Go shootin'; allow reality to teach you........
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Old August 30, 2005, 10:21 PM   #18
ethernectar
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OK, its been a while since I posted this thread, but here's a thought.

On my rifle I use a 20MOA badger base. How do you take this into account when calculating trajectory/ballistics?

Oh, anyone else use this? Its pretty handy: http://ballistics.ntinnovations.com
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Old August 30, 2005, 10:45 PM   #19
Zak Smith
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An inclined scope base can increase the maximum "up" adjustment distance from the primary zero. If the incline amount is greater than the "down" travel with the scope zeroed without the incline base (ie, or approx 1/2 the mechanical travel), it can set a lower limit of where the zero is possible, e.g. you might not be able to get a 100 yard zero, only 200 or more.

But outside of those mechanical concerns, it does not affect the dope or program output.

See http://demigod.org/optics for more details on rifle optics.

-z
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Old September 2, 2005, 03:37 PM   #20
arthurrh
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I'd be willing to bet that your ballistics software is assuming that your scope is parallel to your bore. If it's not, IE from using an inclined base, then all bets are off with it's calculations.
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Old September 2, 2005, 03:41 PM   #21
Zak Smith
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Quote:
I'd be willing to bet that your ballistics software is assuming that your scope is parallel to your bore. If it's not, IE from using an inclined base, then all bets are off with it's calculations.
As I just posted, this makes no diffrence.

For example, if a scope with 60 MOA total adjustment (ie, when it's "centered" it's 30MOA up from bottom) is mounted on a 20MOA base, then it will be zeroed around 10MOA up from the bottom, with 50MOA available up.

The case where is makes a difference is when you have more incline than 1/2 the adjustment distance, e.g. a 40 MOA base on a 60MOA adjustment scope. In that case, you won't have enough "down" adjustment to get a 100 yard zero. You might have to use a 400 yard zero.

-z
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Old September 3, 2005, 08:16 PM   #22
arthurrh
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It would depend on the kind of base. If the base is inclined, not parallel to the bore, it will indeed affect the trajectory, especially at long distances. Most of the basic ballistic software doesn't allow you to compute based on a scope that isn't parallel to the bore.
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Old September 3, 2005, 08:57 PM   #23
ethernectar
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Yeah, still wondering about that. Different guys have different thoughts about the whole thing. Some say it matters, some say it doesn't. The math involved is a bit too abstract for me... None of the ballistic calculators I've seen online take this into account...

One of my buddies pointed out that it could also be because I'm a big guy and perhaps that has some impact on how much the rifle rises as the bullet is fired.

m
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Old September 3, 2005, 09:06 PM   #24
Zak Smith
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Think about it this way---

If an inclined base MADE A DIFFERENCE, then you could not dial a scope 30 minutes of elevation to make hits at 1000 yards.

When you dial the scope's elevation knob, it changes the "line of aim", which is where the reticle appears on the target background.

The whole point of moving the erector assembly (by turning the knobs) to adjust your zero or to dial for elevation is to change the line of aim the reticle specifies while NOT moving the scope tube.

Regardless of what incline the scope base has, once you re-zero the scope at 100 yards, the affect of the inclined base is now taken into account, per the previous examples.


I challenge anyone who thinks it makes a difference to either

a. explain how it makes a difference, beyond what I showed in posts #19 and #21, or

b. do an experiment which proves there is a difference.


Oh, and by the way, I have written a ballistic computer which I use to create all my field range cards. For scoped rifles, the only parameter even close to this is the sight over bore distance.

-z
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Old September 4, 2005, 12:43 PM   #25
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Ethernectar,

Putting your external ballistics program at 105°F will only show the effect of the change of air density on the bullet path. It will not account for the internal ballistic effect of temperature change on powder burning rate, which in turn affects pressure peak timing and muzzle velocity. A load that puts the gun in the wrong part of its muzzle vibration transition can cause small increases in velocity to throw the muzzle disproportionately up or down, depending on the point in the curve (Vaughn, H., 2000, Rifle Accuracy Facts, 2nd Ed., Preceision Shooting Pub., pp. 85-89). Even so, it shouldn't be as great as you are stating.

External ballistics programs basically interpolate for the BC's given, and won't be more precise than these figures are. I have always found my come-up from 300 to 600 yards with a service rifle with iron sights is about 11.5 minutes, while the programs call for more like 13 minutes (168 grain Sierra MatchKing). I have always put this off to the change in sight picture and aim point that distance imposes on the naked eye, but have not verified the error with a scope. It should not be an issue with a scope. This 1.5 minutes difference would be about 6 clicks on your scope.

Soooo... A dumb question. My external ballistics program (QuickTARGET) let's me sepcify the number of inches per click at 100 yards before generating tables. Any chance your's does too, and that you got the wrong number in?

1.000 m.o.a. = 1.0472 inches/hundred yards
0.500 m.o.a. = 0.5236 inches/hundred yards
0.250 m.o.a. = 0.2618 inches/hundred yards
0.125 m.o.a. = 0.1309 inches/hundred yards

Assuming you had that covered, let's get down to nuts and bolts. Please post your load. If you know the water capacity of your cases, post that. Provide your barrel length. Let us calculatory nit-pickers have a go at it!

Also: What kind of chronograph did you use and what is the screen spacing? My dad's Chrony, with its short spacing, consistantly disagrees with my Oehler 35P (with 4 foot spacing and check screen) by about 240 fps in this velocity range.

Nick
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