- Sep 1, 2009
Hum. You got me. I don’t even calculate for spin drift till after 700 yards, and that’s only if I’m trying to shoot a 2” or 3” target. It’s just that the calculations are so small you can’t even adjust for them under 700 yards.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).
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).
View attachment 214123
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.
Here's the 503 yd group. Same protocol, with 9 shots, 3 shot strings, fully cooled between strings.
View attachment 214130
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.
This is the weird part...
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 dialed 6.75 MOA for this group, but it was 0.78 MOA high.
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.
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.
5) I was shooting ESE, but according to Ballistic, Coriolis is only good for ~0.05 MOA at 503 yds.
So...Dubya Tee Eff?!?!