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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 04:42
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With the gong challenge looming on Sat, I went to the rifle range to zero my rifle and re-check my bullet drop.
 
I set up two targets and shot from the 200 and 300 range.
 
At the two hundred I shot one shot using my 100 m zero. Bullet drop = 3and 1/4 inch. I then shot 1 shot at 20X magnification and second reticule which gives me 2MOA. Bullet struck 4inches up from zero shot and a bit high. This is in line with my expectations, so far so good. I crank up to 25X which gives me 1.5 MOA and I am home and dry with three shots.
 
Next up I check my 300 yds zero. My 100 m Zero shot is off the paper, 13 inches high. I go to 20X and use my 3rd reticule at 4,5 MOA.  Again home and dry. I pack up, ready for the Joffe Gong Challenge!!
 
 
 
So what went wrong at the Joffe Challenge? I felt I was shooting well ie no shaking and I had good trigger control etc.
 
After the comp I went back to the 150 200 250 300 and 350 gongs. I adjusted my elevation and hit all the 4 inch gongs with regular monotony.
 
I give you the elevation I used in the comp, followed by the correct elevation.
 
200 m = 3inch/4inch
250 m = 8 inch/11 inch 10 inches
300m = 13 inch/16 inch.
 
So what caused this different trajectory as opposed to my Friday shoot?
 
What was different? Only two things I can think of Temp and elevation.
 
Friday conditions: Temp 30 Deg Celsius and 50ft elevation
Sat Condt: Temp 20 Deg C and 780ft elevation.
 
According to JBM my Friday shoot was the wrong one???? Why???
 
Trajectory Card
Manufacturer: Sierra Description: HPBT MatchKing™ Caliber/Weight: 0.308 in/155.0 gr
Zero Range: 100 m Zero Temperature: 59.0 °F Zero Altitude: 0.0 ft
Zero Height: 0.00 in Zero Offset: 0.00 in
Sight Height: 1.80 in Sight Offset: 0.00 in Line Of Sight Angle: 0.0 deg
Cant Angle: 0.0 deg Wind Speed: 10.0 mph
Show Drop and Windage Deltas: No Windage Correction for Zero Range: Yes
Drop Units: 1.000 in Windage Units: 1.000 MOA
Target Relative Drops: Yes Round Output to Whole Numbers: No
Trajectory Data
50 ft 780 ft
Range (m) 20 °C 30 °C 20 °C 30 °C Range (m)
100 0.0 0.1 0.0 0.1 100
-0.0 -0.0 -0.0 -0.0
125 -0.4 -0.3 -0.4 -0.3 125
0.2 0.2 0.2 0.2
150 -1.2 -1.1 -1.2 -1.1 150
0.4 0.4 0.4 0.4
175 -2.5 -2.3 -2.5 -2.3 175
0.6 0.6 0.6 0.6
200 -4.2 -3.9 -4.1 -3.9 200
0.8 0.8 0.8 0.8
225 -6.3 -5.9 -6.2 -5.9 225
1.0 1.0 1.0 1.0
250 -8.9 -8.4 -8.8 -8.4 250
1.2 1.2
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 07:29
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Chief Sackscratch

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From knowing your a shooter from all your past post and reputation, its easy to  rule out shooter error for sure. I would have to say the elevation may have had the noticeable affect. I am no expert but its the biggest atmospheric change I can see in your data.  I see a small difference (1-2MOA) in POI on my guns when i shoot at the beach 0-10 ft above sea level and then here in Chapel Hill 510ft above sea level
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 07:39
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My level of experience is a bit limited when moving from one range to the next. I had a similiar experience last year when shooting at Calvinia.
 
Unfortunately there is no time or ranges available to range out to 300m etc. At best there is a 100 m range for checking zero.
 
I am definately ruling out shooter error, as I hit the gongs the moment I increased the elevation.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 08:06
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this is part of the reason why i do my load development in the dead of winter. my deer hunting takes place below 32 degrees, and more than likely below 0 degrees. depending on the powder you use temp. obviously can effect results. your looking at temps of what 70 and 85-90 i could see the hotter weather possibly causing some issues, i wouldnt expect to see much of change in that 20c weather.

im inclined to agree with you on the elevation change, if your shots were all higher than you expected at 780ft then thats what it was.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 08:21
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Originally posted by pyro6999 pyro6999 wrote:

this is part of the reason why i do my load development in the dead of winter. my deer hunting takes place below 32 degrees, and more than likely below 0 degrees. depending on the powder you use temp. obviously can effect results. your looking at temps of what 70 and 85-90 i could see the hotter weather possibly causing some issues, i wouldnt expect to see much of change in that 20c weather.

im inclined to agree with you on the elevation change, if your shots were all higher than you expected at 780ft then thats what it was.
 
Pyro, it was actually the other way around. At the coast I experienced less drop then inland. At 3oo yds at the coast the bullet dropped 13 inches, whilst inland it dropped 16 inches. I was going underneath the gongs.
 
 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 08:22
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500 ft of elevation should not drop you 4 inches off the gong IMO. On JBM did you use G1 data or G7? Rework the data with the 2 an see if your difference appears.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 08:24
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higher elevation means thinner air, which equals less drop. if your rifle was sighted in at 50ft above sea level you should have been shooting high. like here where i live im at 1450ft, if i go see my dad and go shoot with him at 7000ft at his range, i cant hit a 2x2 sheet of paper at 100yds. that seems odd.course there is a mile of elevation difference as opposed to your 730ft. that 730ft of difference really only translates to like .02-.03 inches of difference in drop at 300yds.


Edited by pyro6999 - March/08/2010 at 08:34
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 08:41
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Originally posted by pyro6999 pyro6999 wrote:

higher elevation means thinner air, which equals less drop. if your rifle was sighted in at 50ft above sea level you should have been shooting high. like here where i live im at 1450ft, if i go see my dad and go shoot with him at 7000ft at his range, i cant hit a 2x2 sheet of paper at 100yds. that seems odd.course there is a mile of elevation difference as opposed to your 730ft. that 730ft of difference really only translates to like .02-.03 inches of difference in drop at 300yds.
 
Pyro, you are absolutely correct. The targets shot on Friday shows my aimpoints were spot on, yet Sat I was shooting lower.
The missing link is that I did not check zero at 100 yds, which may account for the strange happenings. If I run the JBM with only 0.1 inch below zero at 100 m I come close to the ballistics as per Saturday.


Edited by 8shots - March/08/2010 at 08:45
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 08:44
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Range Drop Drop Windage Windage Velocity Mach Energy Time Lead Lead
(m) (in) (MOA) (in) (MOA) (ft/s) (none) (ft•lbs) (s) (in) (MOA)
0 -1.9 *** 0.0 *** 2850.0 2.509 2795.0 0.000 0.0 ***
50 -0.3 -0.5 0.2 0.3 2741.6 2.414 2586.5 0.059 10.3 18.0
100 -0.1 -0.1 0.8 0.7 2635.8 2.321 2390.6 0.120 21.1 18.4
150 -1.4 -0.8 1.9 1.1 2531.3 2.229 2204.9 0.183 32.2 18.8
200 -4.3 -1.9 3.4 1.5 2428.7 2.138 2029.7 0.249 43.9 19.2
250 -9.0 -3.1 5.4 1.9 2328.4 2.050 1865.6 0.318 56.0 19.6
300 -15.6 -4.5 7.9 2.3 2230.4 1.964 1711.9 0.390 68.7 20.0
350 -24.2 -6.0 11.0 2.7 2134.8 1.880 1568.2 0.466 81.9 20.4
400 -35.2 -7.7 14.7 3.2 2041.5 1.798 1434.2 0.544 95.8 20.9
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 09:23
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was there a large humidity differential?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/08/2010 at 16:40
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At 300m, if humidity was 100% at the coast and 0% at 780ft, you could account for about an inch of drop.  However you have about a 20deg temperature differential which would mean about 1MOA drop, altitude difference would mean about .1MOA rise.  What was the barometric pressure?  That might make up the rest of the drop you are seeing...

 
(for clarification for the 'gotcha crowd'... rise is a relative term here, does not not indicate I think the bullet is actually rising)


Edited by Kickboxer - March/08/2010 at 16:42
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/09/2010 at 02:09
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I doubt if humidity was the cause. Cape Town is exceptionally hot and dry at the moment, similiar to the inland venue.
 
I unfortunately do not know what the barometric pressure was on both days.
 
Thanks for the pointers. This coming Sat I will have a chance to check my zero. Maybe that had shifted, allthough my rifle had suffered no knocks or bumps. I also did not clean it before the Sat shoot.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/09/2010 at 07:20
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Wouter, my point was that humidity is not going to make that much difference.  Depending on which formula set you use, from about 1.5 to 2.5cm at 300 meters.  You can account for about 15-20cm of variation at 1000m due to humidity changes.  High humidity, less drop, low humidity more drop.  For baro pressure... higher pressure, more drop, lower pressure, less drop...
 
Barometric pressure, temperature, altitude... primary POI considerations. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/09/2010 at 08:37
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Kickboxer thanks for the feedback. My target shooting and recording of bullet drop is unfortunately done at one range only. My experience with moving around to different ranges and altitudes is therefore a bit limited and needs brushing up.
 
With hunting we normally zero at 100 m and find small variances. We do make some minor elevation adjustments etc. The target area of most game are big enough to compensate for variances we did not fully take into account.
 
Shooting at 4 inch gongs are a bit more precise though.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/09/2010 at 19:33
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doesn't sound like variance from standard pressure and temp, caused much, although I've seen some weird stuff happen at the dew point with poi changes. thinking here that you may have set the scope and then "chased the shot"  which happens to the best of shooters. Also it looks like each target was up a little higher on the hill, with each range change, although its hard to tell how much.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/09/2010 at 22:37
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Without being there it is very difficult to determine the cause, there are many possibilities.  One COULD be that 8 just had a bad shooting day.  However, the temp change from 30degC(86degF) at zero location to 20degC(68degF) at competition location would result in nearly a 1MOA drop at 300m, which accounts for much of the suspected miss.  A slight drop in pressure could account for the rest.  Or... it could be something completely different.  Results based upon available data are inconclusive...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/10/2010 at 23:34
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If you take in to account the 10 deg C change and my guess would be a change in the pressure, it could make 3 inches of difference at that range.  You may have only gone up 780 ft which isn't much compared to where I hunt and change elevations up to 5000 feet in one day, but with the added change in pressure (assuming it changed) the difference could have been the equivalent to a couple thousand feet.  Makes a difference.

Do you have a reliable weather website that you could check the history for those days and see what the pressure for the area were?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/11/2010 at 02:35
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Helo 18, not sure if I can find the pressures for those days, but I will search the web and also try the flying clubs.
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