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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Riddle me this? Ballistic calc vs reality mismatch...
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<blockquote data-quote="entoptics" data-source="post: 1980036" data-attributes="member: 104268"><p>I'm no newb, but this one has me stumped al little. Went out today to really fine tune my zero and confirm drops before I start hard core "field practice" for the upcoming deer season, and I got a pretty severe disagreement with my ballistic software (Hornady 4DOF, Ballistic, and JBM online calculator).</p><p></p><p><strong>Details...</strong></p><p></p><p>7mm Rem Mag, 175 ELD-X, 1:8 twist, 2900 fps (30 fps ES, 9 fps SD). Bipod and bags, near perfect conditions (other than intense wildfire smoke). 70° with 0-3 mph "thermals" type breeze moving nearly 360° in map direction over the course of fire. Environmental variables changed very little over the 1 hour course of fire (72 to 68° F, 27.56 - 27.58" pressure) , confirmed with Kestrel and iPhone barometer.</p><p></p><p>Here's the 200 yard zero, with 9 shots, fired in 3 shot strings, with full cooling between strings (homemade barrel cooler).</p><p></p><p>[ATTACH=full]214123[/ATTACH]</p><p></p><p>Group is a hair over 1 MOA, which I'm just fine with (and is consistent with dozens of other measured groups). I pulled a couple of these an 1/8 or so, but unlike some statistically challenged folks, I'm not one to quote "If I do my part" or "Lucky they all missed in the same spot" 3 shot groups as the precision my rifle will achieve. This is my hunting rifle, and though I've recorded a few 1/3 MOA, 3-shot groups with it, and several 5 shot 3/4 MOA groups, that's not what we really shoot together. For ethical hunting, I want 9/10 confidence, or even 19/20. That takes big groups, and lots of them.</p><p></p><p>I digress...</p><p></p><p>Here's the 503 yd group. Same protocol, with 9 shots, 3 shot strings, fully cooled between strings.</p><p>[ATTACH=full]214130[/ATTACH]</p><p></p><p>This group is slightly better, though statistically speaking, barely so. Still perfectly happy with it. The offset to the right is most likely spin drift according to 4DOF.</p><p></p><p><strong>This is the weird part...</strong></p><p></p><p>Hornady 4DOF, Ballistic, and JBM online all suggest 6.7 - 6.9 MOA come up from a 200 yd zero (level) to 503 yds with a 2° incline. Taking into account the approximately 1/8 MOA offset in my 200 yd zero, that would equate to pretty much 6.5 - 6.75 MOA.</p><p></p><p><em><u>I dialed 6.75 MOA for this group, but it was 0.78 MOA high.</u></em></p><p></p><p>I've already noticed this high bias before, between 400 and 620 yds (can't go further till it rains a good bit, as there's too much fire danger to be mucking about in the other wheat stubble field). This isn't a one off "weird thing". It's relatively consistent.</p><p></p><p>So...Here's what I know...</p><p></p><p>1) Every shot went past the LabRadar. 2900 fps was average, and right in line with 50-75 other measurements of this load, and if anything, a hair on the slow side.</p><p></p><p>2) I did a box test at 100 yds a couple weeks ago with the scope (12 MOA right shoot 1 round, 20 up shoot, 12 left shoot, 20 down shoot, repeat 3X), and my scope (Sig Whiskey 5, 5-25x50) is within group size error of tracking perfectly.</p><p></p><p>3) I've used my LabRadar and JBM online to verify the BC of the 175 ELDX dozens of times. Values range from 0.680 - 0.720 G1, with an average of ~0.705 (Hornady quotes 0.689 -0.678 G1). With a carefully aimed LabRadar, I get reliable tracking out to 120-200 yds on 7mm projectiles, but usually right around 140 yds. I ran half of the 503 yd shots through the JBM calculator, and they averaged 0.695. Again, a hair on the low side.</p><p></p><p><em><u>4) I ran the BC up in my calculators until I got the right drop at 503 yds (~6.25 MOA), and it requires about a 0.85 BC.</u></em></p><p></p><p>5) I was shooting ESE, but according to Ballistic, Coriolis is only good for ~0.05 MOA at 503 yds.</p><p></p><p>So...Dubya Tee Eff?!?!</p></blockquote><p></p>
[QUOTE="entoptics, post: 1980036, member: 104268"] I'm no newb, but this one has me stumped al little. Went out today to really fine tune my zero and confirm drops before I start hard core "field practice" for the upcoming deer season, and I got a pretty severe disagreement with my ballistic software (Hornady 4DOF, Ballistic, and JBM online calculator). [B]Details...[/B] 7mm Rem Mag, 175 ELD-X, 1:8 twist, 2900 fps (30 fps ES, 9 fps SD). Bipod and bags, near perfect conditions (other than intense wildfire smoke). 70° with 0-3 mph "thermals" type breeze moving nearly 360° in map direction over the course of fire. Environmental variables changed very little over the 1 hour course of fire (72 to 68° F, 27.56 - 27.58" pressure) , confirmed with Kestrel and iPhone barometer. Here's the 200 yard zero, with 9 shots, fired in 3 shot strings, with full cooling between strings (homemade barrel cooler). [ATTACH type="full" alt="IMG_5023.JPG"]214123[/ATTACH] Group is a hair over 1 MOA, which I'm just fine with (and is consistent with dozens of other measured groups). I pulled a couple of these an 1/8 or so, but unlike some statistically challenged folks, I'm not one to quote "If I do my part" or "Lucky they all missed in the same spot" 3 shot groups as the precision my rifle will achieve. This is my hunting rifle, and though I've recorded a few 1/3 MOA, 3-shot groups with it, and several 5 shot 3/4 MOA groups, that's not what we really shoot together. For ethical hunting, I want 9/10 confidence, or even 19/20. That takes big groups, and lots of them. I digress... Here's the 503 yd group. Same protocol, with 9 shots, 3 shot strings, fully cooled between strings. [ATTACH type="full" alt="IMG_5021.JPG"]214130[/ATTACH] This group is slightly better, though statistically speaking, barely so. Still perfectly happy with it. The offset to the right is most likely spin drift according to 4DOF. [B]This is the weird part...[/B] Hornady 4DOF, Ballistic, and JBM online all suggest 6.7 - 6.9 MOA come up from a 200 yd zero (level) to 503 yds with a 2° incline. Taking into account the approximately 1/8 MOA offset in my 200 yd zero, that would equate to pretty much 6.5 - 6.75 MOA. [I][U]I dialed 6.75 MOA for this group, but it was 0.78 MOA high.[/U][/I] I've already noticed this high bias before, between 400 and 620 yds (can't go further till it rains a good bit, as there's too much fire danger to be mucking about in the other wheat stubble field). This isn't a one off "weird thing". It's relatively consistent. So...Here's what I know... 1) Every shot went past the LabRadar. 2900 fps was average, and right in line with 50-75 other measurements of this load, and if anything, a hair on the slow side. 2) I did a box test at 100 yds a couple weeks ago with the scope (12 MOA right shoot 1 round, 20 up shoot, 12 left shoot, 20 down shoot, repeat 3X), and my scope (Sig Whiskey 5, 5-25x50) is within group size error of tracking perfectly. 3) I've used my LabRadar and JBM online to verify the BC of the 175 ELDX dozens of times. Values range from 0.680 - 0.720 G1, with an average of ~0.705 (Hornady quotes 0.689 -0.678 G1). With a carefully aimed LabRadar, I get reliable tracking out to 120-200 yds on 7mm projectiles, but usually right around 140 yds. I ran half of the 503 yd shots through the JBM calculator, and they averaged 0.695. Again, a hair on the low side. [I][U]4) I ran the BC up in my calculators until I got the right drop at 503 yds (~6.25 MOA), and it requires about a 0.85 BC.[/U][/I] 5) I was shooting ESE, but according to Ballistic, Coriolis is only good for ~0.05 MOA at 503 yds. So...Dubya Tee Eff?!?! [/QUOTE]
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Riddle me this? Ballistic calc vs reality mismatch...
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