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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Test results on 264 cal 150 & 160 gr Matrix Bullets
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<blockquote data-quote="BryanLitz" data-source="post: 996221" data-attributes="member: 7848"><p>It depends on how the advertised or estimated numbers were generated.</p><p></p><p>One common scenario is that advertised numbers are based on short range testing (100-200 yards) where the bullet velocity is high. Well, at high speed, the G1 BC is higher, then decays at low speed. The manufacturer advertises a number based on high velocity which isn't <em>wrong</em>, but it's not representative of the bullets performance over long range.</p><p></p><p>Take the 160 grain 6.5mm bullet from Matrix. They advertise a G1 BC of .685. My measurements indicate a G1 BC of .687, but only for velocities over 3000 fps. As velocity slows down, the G1 BC drops, a lot, and over the course of a long range shot, the BC averages more like 0.646, which is the number you'll need to accurately predict drop.</p><p></p><p>This is one reason why the G7 BC is so much better; it doesn't vary as much with speed. So if everyone tested and used G7 BC's, it wouldn't matter (as much) if you tested over 100 or 600 yards, the G7 BC would be more or less the same.</p><p></p><p>I'm not picking on Matrix here, they make good bullets and do their best to provide the most accurate ballistic performance data they can. But to really nail it down takes a serious effort which isn't really practical for the size company they are.</p><p></p><p>So, to answer your question directly, my BC's are quite often lower than estimated or advertised BC's for two reasons:</p><p></p><p>1. Using the G1 BC tested at 'high speed' only, uncorrected for long range decay</p><p>2. Deliberate inflation (probably less common than #1)</p><p></p><p>Most people don't realize how easy it is to estimate BC's with very good accuracy. It's just sectional density divided by form factor.</p><p></p><p>Sectional density is a known, from weight and caliber. A .308 caliber 175 grain bullet has a sectional density of 175/7000/.308/.308 = .264. That's every .308 caliber 175 grain bullet.</p><p></p><p>The other element is form factor, which is related to the drag.</p><p></p><p>Bullets with 'average' drag will have form factors of 1.00, so their G7 BC is equal to their SD.</p><p></p><p>Bullets with 'low' drag will have form factors lower than 1.00, but usually not lower than 0.90.</p><p></p><p>Bullets with 'high' drag will have form factors around 1.10, sometimes much higher but much higher than 1.1 aren't even considered for long range (think of a Barnes TSX flat base or a Swift A-Frame).</p><p></p><p>So looking at any bullet and making a guess about it's form factor being 0.9, 1.0, or 1.1, if you're right, it will get you within 10% of the actual BC.</p><p></p><p>Using the 150 and 160 grain Matrix bullets for example again, the sectional density is:</p><p></p><p>150 grain: 150/7000/.264/.264 = .308</p><p>160 grain: 160/7000/.264/.264 = .328</p><p></p><p>Now we just have to take a guess at the form factors.</p><p></p><p>Based on looking at these bullets (the nose and tail matter most to form factor, length of bearing surface matters little). My guess would be they would have a form factor close to .95. </p><p></p><p>So based on this, the G7 BC's would be:</p><p></p><p>150 (SD/Form Factor) .308/.95 = .324</p><p>160 (SD/Form Factor) .328/.95 = .345</p><p></p><p>As it turned out, the form factors on these bullets were .992 and .989. So the actual BC's are:</p><p></p><p>150 = .308/.992 = .311</p><p>160 = .328/.989 = .332</p><p></p><p>These actual numbers are only 4% different from what was actually tested.</p><p></p><p>Granted I may be better at guessing form factors than average, but with a little practice, and knowing form factors of similar bullets, you can get pretty good.</p><p></p><p>Sectional density gets you most of the way to a good G7 BC, the form factor just modifies it, and you can estimate that pretty close.</p><p></p><p>I often use this as a sanity check for BC claims. You can calculate what the form factor would be for a bullet of that weight, caliber and BC, and decide if it seems reasonable. Often times BC claims infer an unrealistic form factor (like 0.7) for a bullet which look like others having a known form factor of .95.</p><p></p><p>Wow, I didn't mean for this to get so long, just got on a roll.</p><p></p><p>Take care,</p><p>-Bryan</p></blockquote><p></p>
[QUOTE="BryanLitz, post: 996221, member: 7848"] It depends on how the advertised or estimated numbers were generated. One common scenario is that advertised numbers are based on short range testing (100-200 yards) where the bullet velocity is high. Well, at high speed, the G1 BC is higher, then decays at low speed. The manufacturer advertises a number based on high velocity which isn't [i]wrong[/i], but it's not representative of the bullets performance over long range. Take the 160 grain 6.5mm bullet from Matrix. They advertise a G1 BC of .685. My measurements indicate a G1 BC of .687, but only for velocities over 3000 fps. As velocity slows down, the G1 BC drops, a lot, and over the course of a long range shot, the BC averages more like 0.646, which is the number you'll need to accurately predict drop. This is one reason why the G7 BC is so much better; it doesn't vary as much with speed. So if everyone tested and used G7 BC's, it wouldn't matter (as much) if you tested over 100 or 600 yards, the G7 BC would be more or less the same. I'm not picking on Matrix here, they make good bullets and do their best to provide the most accurate ballistic performance data they can. But to really nail it down takes a serious effort which isn't really practical for the size company they are. So, to answer your question directly, my BC's are quite often lower than estimated or advertised BC's for two reasons: 1. Using the G1 BC tested at 'high speed' only, uncorrected for long range decay 2. Deliberate inflation (probably less common than #1) Most people don't realize how easy it is to estimate BC's with very good accuracy. It's just sectional density divided by form factor. Sectional density is a known, from weight and caliber. A .308 caliber 175 grain bullet has a sectional density of 175/7000/.308/.308 = .264. That's every .308 caliber 175 grain bullet. The other element is form factor, which is related to the drag. Bullets with 'average' drag will have form factors of 1.00, so their G7 BC is equal to their SD. Bullets with 'low' drag will have form factors lower than 1.00, but usually not lower than 0.90. Bullets with 'high' drag will have form factors around 1.10, sometimes much higher but much higher than 1.1 aren't even considered for long range (think of a Barnes TSX flat base or a Swift A-Frame). So looking at any bullet and making a guess about it's form factor being 0.9, 1.0, or 1.1, if you're right, it will get you within 10% of the actual BC. Using the 150 and 160 grain Matrix bullets for example again, the sectional density is: 150 grain: 150/7000/.264/.264 = .308 160 grain: 160/7000/.264/.264 = .328 Now we just have to take a guess at the form factors. Based on looking at these bullets (the nose and tail matter most to form factor, length of bearing surface matters little). My guess would be they would have a form factor close to .95. So based on this, the G7 BC's would be: 150 (SD/Form Factor) .308/.95 = .324 160 (SD/Form Factor) .328/.95 = .345 As it turned out, the form factors on these bullets were .992 and .989. So the actual BC's are: 150 = .308/.992 = .311 160 = .328/.989 = .332 These actual numbers are only 4% different from what was actually tested. Granted I may be better at guessing form factors than average, but with a little practice, and knowing form factors of similar bullets, you can get pretty good. Sectional density gets you most of the way to a good G7 BC, the form factor just modifies it, and you can estimate that pretty close. I often use this as a sanity check for BC claims. You can calculate what the form factor would be for a bullet of that weight, caliber and BC, and decide if it seems reasonable. Often times BC claims infer an unrealistic form factor (like 0.7) for a bullet which look like others having a known form factor of .95. Wow, I didn't mean for this to get so long, just got on a roll. Take care, -Bryan [/QUOTE]
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Test results on 264 cal 150 & 160 gr Matrix Bullets
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