Looking for Wisdom at 1000 yards

[Jerrrrstanley]

I went out plinking yesterday and had set up to shoot out to 1200 yards. The cows were gracious enough to ruin that shot by knocking over that target and being in that area after I left. So, I found a rock at an unkown distance, took a guess at 950 yards, dialed 32 minutes and took a spotter shot. I was using the Shooter app and it has done good out to 550 but I had never tried it further than that.
The spotter shot was way low so I cranked 8 more minutes took another spotter shot and cranked 2 more putting me up to 42 minutes. We started pulling dust off the rock and took about 20 shots between the 2 of us there. Looking back at my range card from the Shooter app at 42 minutes was 1150 yards so I thought I had just severly misjudged the distance.
I get home last night, hit up Google Earth, and measure out our shot.... 980 yards. WTF! Double checked my dope in my data book, ran the numbers on JBM and came up with 34 minutes as my solution at 980. Anyone have any advice? Is 42 minutes of elevation normal for a 308 at 1000?
I am shooting a Sig Tactical 2 in 308. Shooting Federal Gold Match 168g.

 

[Jerrrrstanley]

View from our posistion. Just to help put you there hahaha

 

[Rthur]

I don't know what elevation you are shooting at but I'll guess the 168's are beyond their effective range. They can start doing interesting things around 8-850 if you don't have thin enough air.

R

 

[Jerrrrstanley]

3200 feet, 80 Degrees, 2-4 mph wind from the 7 o'clock. I wish I had taken a picture of the rocks. Below the white rock we were shooting at was a 2 feet x 3 feet rock face hidden in the weeds from our posistion. Minus the spotters we shot about 15 rounds at the white rock. About 8 or 9 were acounted for on the rock face in a 15" group. We had 3 hits on the white rock.
I am thinking either I fudged up and only dialed 32 and thought I dialed more or my scope isn't tracking correctly. My employer bought us remanufactured Leupolds for our these rifles.

 

[hessian]

My load of 42 gr. of RL15 and 168 Amax has a change of 41 min. from 650 to 1150.About 2200 ft.I have excellent results with this load at 1150. No keyholing.

 

[Darkphage]

I have found that it is pretty hard to be very accurate with google earth. Also, at that range judging distance becomes almost impossible. Misjudging by a couple hundred yards or more @ 1000+ is commonplace -- even for very experienced shooters. Other than shooting an unknown distance, do you know your muzzle velocity? That will have a huge impact, also stated BC for any given bullet it just an average. It will vary a little based on barrel twist, barrel length ect. Sometimes stated BC needs to be tweaked to match observed data, but that should only ever be done if shooting solid known distances, with accurate MV and up to date atmospheric conditions. To answer your question, I can think of several things that "may" have affected things, but it is impossible to be certain. I still think the most likely candidate to give you data you weren't expecting is still imprecise range estimation.

Good Luck,

Darkphage

 

[Jerrrrstanley]

I have been thinking along your lines. There are too many unknowns to try to guess what may be the problem. I'm going to head back to the same spot and get a GPS reading at the target so I can know the distance for certain. I'm going to shoot the same distance again and double check everything. It will be a while now because I just found I have a broken freaking heel. I guess I could find someone to walk the mountain for me and I can drive to my firing position. Hmmmm... hahaha

 

[woostri]

Biggest source of error at that range is whether or not the ballistic coefficient is correct and whether your muzzle velocity is what you think it is. I can assure you that the shooter app is accurate within inches even at extreme ranges. The problem is the input data is subject to error so that the output you get is not real. The most accurate BC I know of for this bullet is published in Bryan Litz's book, applied ballistics for long range shooters. He actually measured BC's for a ton of bullets when putting that book together. It shows 0.218 for G7 and 0.427 for G1. Shoot has the Sierra 168gr Matchking (Litz) that uses his published BC, it should be dead nuts on assuming you give it the true muzz vel and weather conditions....

Ahh, the other biggest source of error. Scope? I cant tell in the picture, but could be that the adjustments on your scope are not true. I like doing box testing on any new scope just to eliminate that source of error.

According to my Shooter app, your muzzle vel would have to be down around 2400 to get 42moa at 980 yards with a 200 yard zero....interesting.

 

[RobertB]

42 MOA for some rifle/loads isn't uncommon. Even more so when shooting a 168 SMK. I'm at sea level right now and things can get crazy quick here in certain weather conditions with a 168 SMK at longer ranges.