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November 3, 2015, 02:55 PM | #1 |
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What does this ballistic chart mean?
I still can't get my arms around in the difference between the two different choices for drop: "in/100yd" column versus just plain "inches" on the JBM ballistics table. Can someone explain? Thanks.
Calculated Table Range Drop Drop (in/100y) (in) 0 *** -1.6 25 -1.0 -0.3 50 1.0 0.5 75 0.8 0.6 100 -0.0 -0.0 125 -1.1 -1.4 150 -2.4 -3.7 175 -3.9 -6.9 200 -5.6 -11.
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November 3, 2015, 03:07 PM | #2 |
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Most scopes the adjustments are in fractions of an inch at 100 yards, sometimes called "Shooter's MOA". A true Minute of Arc is 1.047" at 100 yards.
So for that table, if you are want to change your zero from 100 to 200 yards, you will go up on the scope 5.5 inches, (for 1/4" clicks, 22 clicks), it will move toe point of impact 11 inches at 200 yards. Clear as mud? |
November 3, 2015, 06:17 PM | #3 |
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It often helps me to plot the data to get a 'picture' on what's going on. The plot is of the 1st and 3rd columns of the table--drop vs range:
The first 'Drop' column in the table (column 2), is given in inches per 100 yds, and is a drop rate...how fast is the bullet dropping for every 100 yards it travels at the given range. [This isn't correct. See the correct interpretation below. My bad.] The second 'Drop' column is the actual amount of the drop...how far did the bullet drop at the given range. The negative numbers just mean the opposite of drop--rise. These numbers are relative to some imaginary zero-line, or line of sight. The chart horizontal axis is in yards, the vertical axis is in inches. Tried 3 times to make it bigger--sorry.
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November 4, 2015, 07:19 PM | #4 | |
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Quote:
The in/100y is simply that, the total drop divided by the range in 100 yard increments. To keep the math easy, let's look at 1000 yards, running the numbers for a .264 AMAX at 2808 FPS with a 200 yard zero. Range Drop Drop (yd) (in) (in/100y) 0 -1.8 *** 100 1.5 1.5 200 0 0 300 -6.9 -2.3 400 -19.7 -4.9 500 -39.1 -7.8 600 -65.8 -11 700 -100.6 -14.4 800 -144.6 -18.1 900 -198.7 -22.1 1000 -264.3 -26.4 1100 -342.9 -31.2 1200 -436.1 -36.3 1300 -546.2 -42 1400 -675.5 -48.2 1500 -826.9 -55.1 If your "Drop Rate" answer was correct, the in/100 yards figure would be -66, as that is how far the bullet dropped in the previous 100 yards, but it isn't. What is is, is the total drop in inches, 264.3 divided by the range in 100 yard increments, 10, giving a in/100 yard figure of 26.4. The same is true at every yardage on the table. 700 yards, total drop 100.6÷7= 14.37, rounded to 14.3 by JBM. 1300 yards, total drop 546.2÷13= 42.01, rounded to 42 by JBM. |
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November 4, 2015, 07:29 PM | #5 |
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The bullet drop distance is usually based on a barrel parallel to the ground; the drop is then the amount the bullet drops below the barrel axis line at some given range.
Other tables assume a rifle zeroed at some given distance, usually 100 yards, and show the distance above the barrel axis at a shorter range and below the axis at a greater range. The barrel line (the bore axis extended indefinitely) is NOT the same as the line of sight, though it could be (rarely is) parallel to it. Jim |
November 4, 2015, 07:35 PM | #6 | ||
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Quote:
Quote:
A drop of 11 inches at 200 yards can be converted to a drop at some other distance. I can do that with a very simple (and incorrect-but-close enough) linear conversion--"if it drops 11 inches at 200 yards, then at half that distance it will have dropped half as much", or, 5.6" inches. No, 5.6 isn't half of 11...let's let that go for now. No, the drop won't really be half as much--let that go, too. So, any actual drop at any actual distance can be converted to a fictional drop at some other distance by simply scaling the numbers. But, I choose to scale the drop to 100 yards, specifically. Why? Because x inches per 100 yards is x minutes of angle (roughly--close enough). And why do I care about minutes of angle? It's a useful constant that is independent of range--
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November 5, 2015, 10:00 AM | #7 | |
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Quote:
If the bore were parallel to the ground for either column the drop would never go positive & the example has +ve numbers in both columns.
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November 5, 2015, 10:40 AM | #8 |
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Bullet path
What about the bullet path column?
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November 5, 2015, 02:19 PM | #9 | |||
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Quote:
Quote:
What's the question about it, though? Quote:
How useful are such tables if your gun isn't zeroed at the same range the tables are? Can they still be valuable if, say, you've zeroed at 100 yards but the table is zeroed at 200? Not sure there'd be much point since the mental math would take me longer than just re-zeroing the gun.
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November 5, 2015, 06:20 PM | #10 |
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You have to remember that most sighted in barrels point (slightly) upwards. By how much depends on multiple variables like sight height velocity Bullet's BC & so on.
Because there's a slight upward angle there's also a slight amount of vertical thrust. So you have to factor in the changing balance between decreasing vertical thrust (worked out by a triangle of velocities) & increasing gravitational pull.
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November 5, 2015, 07:11 PM | #11 | |
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Quote:
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November 5, 2015, 07:26 PM | #12 |
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bullet path
When I first started using ballistics tables/charts, I was confused by the columns called bullet drop and bullet path. When using a ballistics table to determine the number of inches a bullet will drop at a given range, should I use drop or path?
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November 5, 2015, 10:04 PM | #13 | |
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Quote:
http://www.jbmballistics.com |
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November 5, 2015, 10:34 PM | #14 | |
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Quote:
But I don't see what this has to do with reading a ballistic chart, which is all the op wanted to understand. As for 'drop' and 'path', in the tables I've seen at least, the term 'drop' refers to the distance the bullet has moved away from the bore axis...think in terms of the comment made above regarding tables that assume the gun barrel is perfectly horizontal. This value has nothing at all to do with line of sight, zeroing the gun or any other aspect of sighting in a gun or hitting a target, near as I can tell. It's just how far the bullet drops, at a given distance from the muzzle, if the bore is parallel to the ground. 'Path' is the distance, again, at a specified range, between the bullet and the line of sight. It's the only metric that has much meaning, as far as I know, when it comes to aiming, 'sighting in' or zeroing. At least this is how I understand it. Where the bullet it going to hit (that is, where we predict it's going to hit) a target is only of interest in the context of where the gun is aimed (line of sight), and that is what path represents. So, 'path' would be the one you use, as 'drop' has no practical use I'm aware of when it comes to shooting a gun. 'Drop' is something that would be useful for comparing the performance of bullets, as it directly reflects the identical circumstances of bullet launch--there's no complication of the bullet being aimed upward to increase range (and change trajectory). So, you're comparing apples-to-apples, so to speak.
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November 5, 2015, 10:56 PM | #15 | |
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November 6, 2015, 12:13 AM | #16 |
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I hope you mean that the 'drop' information is useless, as opposed to knowing what it is when you see it in a table being useless. I sure can't imagine how it could be used for any practical purpose--if you have the knowledge, means and the model to calculate drop, then I think you already have all the information you need to assess whether one bullet will have a flatter trajectory than another. Why you'd want or need to plot it or publish it I just don't know.
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November 6, 2015, 01:25 AM | #17 | |
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November 6, 2015, 08:45 AM | #18 |
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Bullet path
So path is the column to use when you have determined the range to a target, and want to make a change to the scope elevation or hold over, to correct for the number of inches a bullet will drop in flight from the muzzle to target. I've noticed a big difference between the numbers in the drop and the path columns.
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November 6, 2015, 10:36 AM | #19 |
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Its a mathematical expression used to divide angled thrust into its component parts for calculation.
Example a 45/45/90 triangle has a 50:50 split between the 2 sides bounds by the hypotenuse. a 30/60/90 has a 2:3 ratio.
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November 6, 2015, 11:37 AM | #20 | |
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November 6, 2015, 03:26 PM | #21 |
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Lets try a slightly different approach.
This is a chart of the 2 sets of numbers, each is color coded to its data. I read the 2 sets as "This is the actual drop from your 200 yard zero" but at a distance of 100yds. These are the MAGENTA numbers & tracks. "This is the amount below the line of sight you bullet will be at theses distances with a 200 yd zero" These are the Blue colored numbers & tracks (Note although it says "100 yd" the actual "0.00" is at the 200 yd range in both tables.) (Images (C) Wogpotter 2015) Why have both sets? Easy! For different methods of aiming off for distances other than the zeroed 200yds where the path is 0.00" in both cases. If you're gong to adjust your Point of aim by clicking the scope knobs this is how much you change it (with a 1/4" click for 100 yds you multiply by 4@ 100 2 @ 200 & so on.) That's the BLUE numbers. If instead of twiddling you aim off by the opposite amount then you use the Magenta ones. Rephrased as: "To hit dead on with your 200 yd zero aim 39" high at a distance of 500 yds.
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November 6, 2015, 04:16 PM | #22 |
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1st column: range in yards.
2nd column: MOA equivalent of 3rd column. 3rd column: poi relative to poa. They may be named differently, but that is what they really are. BTW, the sight is 1.6" above bore axis in this gun. -TL |
November 7, 2015, 09:40 AM | #23 |
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Ballistics chart
In reference to post #18
This is an example of the chart Remington, REM Shoot puts out! I am thinking the 'path' column is the one I need to use. |
November 7, 2015, 10:28 AM | #24 |
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Yep, Path is what you want. Drop is pretty much useless.
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November 7, 2015, 10:49 AM | #25 |
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Here's a very simple explanation that might help: http://home.insightbb.com/~bspen/ballistics.html
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