Range calculation problem

G

Gandhi

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Location
Lower Mainland of BC
Hello folks,
I'm having this odd problem with my ranges. To start:
- Rem 700 .308
- Nightforce 8-32x56 NXS NPR-2
- sight height = 6.1235cm
- Hornady 178gr BTHP w/ 45.5gr of W-760 Federal 210M in Hornady brass
- G7 BC = 0.257
- Chrony at 2563fps (781m/s) on November 16, 2013 and 2504fps (763m/s) on December 8, 2014 at 15°C
- Zero @ 100 meters; LOC Port Coquitlam, BC @ 300m ASL

After having checked my MVs, I used JBM ballistics to produce a series of ranges, in 10m incriments, out to 1200m. I have proven that 470m at a calculated drop of 12.7MOA is right on the money; 21 consecutive rounds marked on a 12in target. However, the sheet predicts a 630m drop to be 21MOA and I have landed on 16.5MOA on a 6in target; which suggests a range of 550m. A difference of 4.5MOA produces an approximate drop of 27 inches at this range. The only real thing I can think of is that there is an error in the ranging of the 630m target; however, I've had two other shooters mark out the same target to be the same distance (you know who you are!)

Any input would be AWESOME!
Cheers!
 
I had similar happenings past 600m (shooting high) before I started using density altitude as a correction. I am in BC as well and the DA can very quite a bit depending on conditions. The warmer it gets, the less dense the air. Using DA fixed my drop chart inaccuracies at longer range.
 
Confirm the distance the scope actually moves per click or minute.

Many scopes say they are such and such per click but travel more or less. This seems a bit extreme given the scope but you never know.

Compare the ranging with another quality rangefinder. Most rangefinders from better brands have very little error at these distances.

Have you considered using another chronie?

Jerry
 
it has to be your ranging. I have a similar (albeit not identical) load
2554 FPs measured by chrony slinging 168 smks out of my 308.

for comparisons sake
you, 470 meters, is 12.7 MOA (or 3.7 Mil)
me, 456 Meters, is 3.6 Mil ( 12.4 MOA)

So we are fairly close out to that distance.

My sight setting for 640 meters is 6.4 mils (or 22 MOA), so your calculation is about right. I put the confusion on the ranging of the target. At that distance, were you maybe picking up the backstop rather then the target?
 
Hey guys! Thanks for the quick replies! I'm looking at getting a Kestrel sometime so I can take more accurate environmental readings, as this obviously makes a dramatic difference in drop calculations.

Thanks again!
Cheers!
 
mustang979 said:
it has to be your ranging. I have a similar (albeit not identical) load
2554 FPs measured by chrony slinging 168 smks out of my 308.

for comparisons sake
you, 470 meters, is 12.7 MOA (or 3.7 Mil)
me, 456 Meters, is 3.6 Mil ( 12.4 MOA)

So we are fairly close out to that distance.

My sight setting for 640 meters is 6.4 mils (or 22 MOA), so your calculation is about right. I put the confusion on the ranging of the target. At that distance, were you maybe picking up the backstop rather then the target?
Click to expand...

I've tried three seperate calculators:
1) AE ballistic
2) Strelok Pro
3) JBM Ballistics

All come up with the same range calcs. So there must be something up with the ranging method. I'll give that a try!

Thanks!
 
Dude, the gong is 630 yards.
I'm pretty sure who this is, you got the funny shaped barrel, right? ;)

I just ran all your numbers on iSnipe, at 1500ft elevation, today's pressure and humidity, for 630 yards I get 14.9 minutes.

Regardless, using your actual "up" you can reverse engineer for velocity. Back it up at another know distance.
 
9x19p said:
Dude, the gong is 630 yards.
I'm pretty sure who this is, you got the funny shaped barrel, right? ;)

I just ran all your numbers on iSnipe, at 1500ft elevation, today's pressure and humidity, for 630 yards I get 14.9 minutes.

Regardless, using your actual "up" you can reverse engineer for velocity. Back it up at another know distance.
Click to expand...

ya, you might be right.

I plugged in what was at the top there, didnt double check anything so I could be off but i came up with this:
moa.jpg


I am not familiar with this ballistic calculator, but looks close to what Ghandi had. what did i do wrong here?

edit: because 470m would be around 513 yards. sorry for confusion.
 
9x19p said:
Dude, the gong is 630 yards.
I'm pretty sure who this is, you got the funny shaped barrel, right? ;)

I just ran all your numbers on iSnipe, at 1500ft elevation, today's pressure and humidity, for 630 yards I get 14.9 minutes.

Regardless, using your actual "up" you can reverse engineer for velocity. Back it up at another know distance.
Click to expand...

OOOHHHHHHHH! HA! Boy do I feel like a dumbass! Thanks 9x19p! HAHA!
 
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