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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
? about ballistic programs
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<blockquote data-quote="BryanLitz" data-source="post: 278655" data-attributes="member: 7848"><p>I believe that the scope adjustments are usually responsible for 'field data' not matching 'ballistic program' data.</p><p></p><p>It's true that published BC's are not always accurate, sometimes they're off by up to 10%, 5% is more common. To a smaller degree the barrel you're shooting the bullet from may affect the BC too, but only by 1 or 2% max.</p><p></p><p>When someone observes that they need a BC that's 30% or 50% higher than advertised to get their field data to match the program, there's probably something else at play. I think in most cases it's understanding the scope adjustments.</p><p></p><p>For example, we all use 1/4 MOA clicks, right? So if you need 16 clicks to hit your point of aim at 400 yards, that's 4 MOA * 4 * 1.047 = 16.75" of drop at 400 yards, right? Now you plug in all the variables into the ballistics program and find that the bullet you're shooting requires a BC of 0.800 to match your <em>observed</em> drop. What if that bullet has an advertised BC of 0.400? Is it possible that bullet is just twice as good from your rifle? Probably not.</p><p></p><p>Go back to the part where we calculated drop from clicks. We arrived at 16.75" by applying the <em>assumption</em> that the crosshairs moved exactly 1/4 MOA per click as advertised. <em>Has this been verified</em>? What if the crosshairs actually move .265 MOA per click instead of .25 MOA? In that case the actual scope adjustment for 16 clicks would be 16*.265 = 4.24 MOA, which correlates to 17.75" of drop at 400 yards (instead of 16.75"). This is enough to cause an error of about 15% in a <em>derived</em> BC if you aren't aware of the error in scope clicks.</p><p></p><p>Furthermore, if the target was actually 393 yards or 407 yards, that could also cause a 1" error in drop and the same 15% error. And, can your rifle really group well enough to resolve 1" of drop at 400 yards?</p><p></p><p>This isn't an attack. There are a lot of things to think about when you try to derive BC's from drop data, and I'm just illuminating some of them in response to the question asked. The calculation is very sensitive to error.</p><p></p><p>Let's say that you find a number that you can plug into the BC field of a ballistics program that results in a predicted trajectory that matches up to your observed drop (using scope adjustments). If you don't know the exact, true value of your scope adjustments, the number you're using isn't a BC, it's just a number that you input which causes output to be generated which happens to offset the error in your scope clicks. If a bullet has an advertised BC of .4, and you need to input .6, there's probably something wrong.</p><p></p><p>So what if something is wrong? Who cares? The point is to be able to calculate a trajectory that will enable you to hit targets with <em>your</em> scope, right? Who cares if it's not the <em>actual, true</em> BC, it works. Well, kinda. </p><p>It can work for you and your particular scope to predict drop, but the retained velocity (important for knowing when the bullet goes subsonic), kinetic energy (important for lethality considerations), and wind deflection will all be very wrong. By accepting the use of a BC that's not the true BC, you're flushing all the other output generated by the program down the toilet. </p><p></p><p>Furthermore, no-one else will necessarily be able to use <em>your</em> BC. Not because the bullet actually flies different from the two rifles, but because the rifles have different scopes, which move different amounts when '30 MOA' is dialed in. One scope might move 29 MOA, the other might move 31 MOA and the two shooters, if not aware of their scope adjustment error, would <em>perceive</em> different drops (different BC's) from their rifles even though the bullet may actually flies exactly the same from both.</p><p></p><p>Also, if you've arrived at a BC with a ballistics program and you didn't input the measured atmospheric conditions (air temp, pressure, humidity), then the BC you arrive at is also not useful to anyone else, or even for yourself on any other day.</p><p></p><p>If you've arrived at a BC from observed drop data, and you haven't checked your chronograph against at least one other unit, then that's a potential source of error. Chrono's are usually good at measuring shot-to-shot variation (which is important for developing loads) but if they have a systematic bias (ie, always reads 30 fps slow for example) then any BC you derive assuming you know the muzzle velocity will be wrong.</p><p></p><p>The range to the target needs to be known exactly. If your laser rangefinder says 1007 yards, and you treat it as 1000 yards, you will have error.</p><p></p><p>Some of the above error sources are more picky than others, but they can all add up to a <em>perceived</em> BC that's very far from the <em>actual</em> BC. This may be fine if you're just looking for a <em>number</em> that helps you hit targets with your specific scope, but unless you've measured your scope adjustment and taken all the other steps to remove error from your test, then all you can say is that it's a number, not a BC.</p><p></p><p>-Bryan</p></blockquote><p></p>
[QUOTE="BryanLitz, post: 278655, member: 7848"] I believe that the scope adjustments are usually responsible for 'field data' not matching 'ballistic program' data. It's true that published BC's are not always accurate, sometimes they're off by up to 10%, 5% is more common. To a smaller degree the barrel you're shooting the bullet from may affect the BC too, but only by 1 or 2% max. When someone observes that they need a BC that's 30% or 50% higher than advertised to get their field data to match the program, there's probably something else at play. I think in most cases it's understanding the scope adjustments. For example, we all use 1/4 MOA clicks, right? So if you need 16 clicks to hit your point of aim at 400 yards, that's 4 MOA * 4 * 1.047 = 16.75" of drop at 400 yards, right? Now you plug in all the variables into the ballistics program and find that the bullet you're shooting requires a BC of 0.800 to match your [i]observed[/i] drop. What if that bullet has an advertised BC of 0.400? Is it possible that bullet is just twice as good from your rifle? Probably not. Go back to the part where we calculated drop from clicks. We arrived at 16.75" by applying the [i]assumption[/i] that the crosshairs moved exactly 1/4 MOA per click as advertised. [i]Has this been verified[/i]? What if the crosshairs actually move .265 MOA per click instead of .25 MOA? In that case the actual scope adjustment for 16 clicks would be 16*.265 = 4.24 MOA, which correlates to 17.75" of drop at 400 yards (instead of 16.75"). This is enough to cause an error of about 15% in a [i]derived[/i] BC if you aren't aware of the error in scope clicks. Furthermore, if the target was actually 393 yards or 407 yards, that could also cause a 1" error in drop and the same 15% error. And, can your rifle really group well enough to resolve 1" of drop at 400 yards? This isn't an attack. There are a lot of things to think about when you try to derive BC's from drop data, and I'm just illuminating some of them in response to the question asked. The calculation is very sensitive to error. Let's say that you find a number that you can plug into the BC field of a ballistics program that results in a predicted trajectory that matches up to your observed drop (using scope adjustments). If you don't know the exact, true value of your scope adjustments, the number you're using isn't a BC, it's just a number that you input which causes output to be generated which happens to offset the error in your scope clicks. If a bullet has an advertised BC of .4, and you need to input .6, there's probably something wrong. So what if something is wrong? Who cares? The point is to be able to calculate a trajectory that will enable you to hit targets with [i]your[/i] scope, right? Who cares if it's not the [i]actual, true[/i] BC, it works. Well, kinda. It can work for you and your particular scope to predict drop, but the retained velocity (important for knowing when the bullet goes subsonic), kinetic energy (important for lethality considerations), and wind deflection will all be very wrong. By accepting the use of a BC that's not the true BC, you're flushing all the other output generated by the program down the toilet. Furthermore, no-one else will necessarily be able to use [i]your[/i] BC. Not because the bullet actually flies different from the two rifles, but because the rifles have different scopes, which move different amounts when '30 MOA' is dialed in. One scope might move 29 MOA, the other might move 31 MOA and the two shooters, if not aware of their scope adjustment error, would [i]perceive[/i] different drops (different BC's) from their rifles even though the bullet may actually flies exactly the same from both. Also, if you've arrived at a BC with a ballistics program and you didn't input the measured atmospheric conditions (air temp, pressure, humidity), then the BC you arrive at is also not useful to anyone else, or even for yourself on any other day. If you've arrived at a BC from observed drop data, and you haven't checked your chronograph against at least one other unit, then that's a potential source of error. Chrono's are usually good at measuring shot-to-shot variation (which is important for developing loads) but if they have a systematic bias (ie, always reads 30 fps slow for example) then any BC you derive assuming you know the muzzle velocity will be wrong. The range to the target needs to be known exactly. If your laser rangefinder says 1007 yards, and you treat it as 1000 yards, you will have error. Some of the above error sources are more picky than others, but they can all add up to a [i]perceived[/i] BC that's very far from the [i]actual[/i] BC. This may be fine if you're just looking for a [i]number[/i] that helps you hit targets with your specific scope, but unless you've measured your scope adjustment and taken all the other steps to remove error from your test, then all you can say is that it's a number, not a BC. -Bryan [/QUOTE]
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