So of course I'm familiar with the angle cosine calculation, but I've found that the math does not seem to translate to reality. I found myself a nice isolated 750 yard stretch where the target is at about a 5 degree angle vs. the firing point. Based on my angle cosine table, only at about 20 degrees should there be a significant affect on range adjustment. However, shots today showed a full 4 moa decrease required. Ballistic calculator called for 24 moa with 100 yard zero, and the reality was about 20 moa. There was a 20 km/h tailwind, but I doubt this contributed much. I suppose next on the list is to confirm 100% my muzzle velocity with a chorograph. Tables and dope at shorter range indicates 2538 fps with 178 gr hornady bthp pushed with 46.9 gr of h414. I can't imagine a significant coriolis effect at only 750 yards? FYI I was shooting from west to east, so it's possible there was a small effect.
effect of elevated target on range adjustment
- Thread starter joeblow38
- Start date
Tailwind will drop a bullet less then still air! Headwind wil drop a bullet more then still air.
I plugged your info into the ballistic calculator I use. The difference between a headwind and tailwind of 20 km/h makes a difference of 0.2 MOA at 750 yards. That doesn't appear to be your issue.
Probably contributed .2 MOA like your ballistic calculator said. The rest was other variables not properly accounted for or observed.
joeblow38;
Although you made a suggestion that it may not be a coriolis effect at750 yards, it;s true that it may have a very small impact at that distance. However,you did state that you were shooting west to east where the coriolis effect comes into play.
I have attached a youtube from Gunwerks segment where the coriolis issue is explained in length. I hope that it helps for some members to understand what the terminology "coriolis" is all about.
Although you made a suggestion that it may not be a coriolis effect at750 yards, it;s true that it may have a very small impact at that distance. However,you did state that you were shooting west to east where the coriolis effect comes into play.
I have attached a youtube from Gunwerks segment where the coriolis issue is explained in length. I hope that it helps for some members to understand what the terminology "coriolis" is all about.
Sorry to hear that
, since it isn't useful:http://www.gun-blog.com/2011/11/cosine-rules-for-uphill-downhill.html
http://www.gun-blog.com/2012/04/cosines-are-useless-for-uphilldownhill.html
Although you are right to think 5 degrees doesn't do much at 750 yards.
Coriolis might be a couple of inches at 750 yards, not several moa.
So probably something else, yes.
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
Reading between the lines, it sounds like the 24MOA you expected is based on an estimated (not measured) muzzle velocity, and you did not have a prior 750y known elevation from firing that scope+rifle+ammo at that distance. Nothing wrong with this, and it's what you should put on your scope to fire your first shot, but the "24MOA" calculated is only as good as the various errors that are incorporated in its calculation:
- actual M.V. versus predicted/estimated
- at 750y, you're starting to be far enough away that it would be useful to use a good measured G7 b.c. (i.e. Litz's) rather than a manufacturer's published G1 numbers
- Was your actual air temperature and altitude used in your calc'n?
- how big are the "minutes" on your scope? I have seen "scope minutes" as small as 0.8MOA and as large as 1.1MOA.
- what was the actual distance? (lasered to 750y, taped off, estimated, read from googlemaps, etc?)
The very small factors in the ballistics solution, which are almost certainly not measurable or meaningful in this scenario, include:
- the 5 degree slope (Cos 5 degrees = 0.996 --> therefore 750 yards is equivalent to 747 yards)
- Coriolis
- humidity
- spin drift
- head/tail wind
According to calculations, coriolis could contribute about 2 inches, and the wind, worst case was about 30 km/h tail, so say 2 inches. So that's 0.5 moa accounted for. I agree the 5 degree incline should contribute minimally. So it must be muzzle velocity that's the biggest factor. Note I'm now at 40 in pounds of action screw torque, and using the new 20 moa rail. I was hitting about 0.3 moa on 100 yard site in, so looks like a good node with my existing load. Not sure if this could contribute. I agree I should do a tracking test to confirm the clicks are very close to 0.25 moa; pretty sure they are. I only had time to fire 2 shots @ 750 yds. The group was 1 moa high (scope was @ 21 moa past 100 yard zero based on previous attempt @ 24 moa where shots overshot the gong altogether). The 1 moa right was due to 9 o clock wind that was gusting. Therefore predicted correct elev. adjustment is 20 moa. Note I measured the range with gps and with mildot; and they both came back with 750 yards. However, gps measures distance over land, not the range as the crow flied. The first half of the range is flat, then the land inclines about 10 degrees to the target. Therefore the land distance is greater than the line of sight distance to the target. I plugged in 725 yards into jbm, and it comes back with 23 moa, which is still only 1 moa less than 750 range calc. I measured the line of sight range very carefully with the mildot, and so I'm sure I'm not far off 750 yards. There's no way it's 700 yards, which would still result in 22 moa.
So to conclude, somehow the muzzle velocity must be the issue here, assuming good clicks on the scope.
Upon inputting 2650 fps (100 fps faster than indirectly measured mv based on previous drops), with 180 degree wind @ 20 mph (worst case gust), temp of 40 F, alt of 100 ft., the output is 21.5 moa. This does not take into acct coriolis (which I don't see as an input on jbm). So I could take off 0.25 moa only for that, which is not significant. It would be about 2 inches @ 750 yards.
I guess a chronograph is next on the list.
So to conclude, somehow the muzzle velocity must be the issue here, assuming good clicks on the scope.
Upon inputting 2650 fps (100 fps faster than indirectly measured mv based on previous drops), with 180 degree wind @ 20 mph (worst case gust), temp of 40 F, alt of 100 ft., the output is 21.5 moa. This does not take into acct coriolis (which I don't see as an input on jbm). So I could take off 0.25 moa only for that, which is not significant. It would be about 2 inches @ 750 yards.
I guess a chronograph is next on the list.
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
Confirm actual muzzle velocity.
Confirm the actual distance your scope is moving. Some scopes are seriously wonky through their travel.
Confirm the range and ambient conditions. Add the G7 BC to the ballistics calculations.
Odds are it is the scope. But if your target needs "24 up", so be it.
As long as it repeats, it's all good.
Jerry
Confirm the actual distance your scope is moving. Some scopes are seriously wonky through their travel.
Confirm the range and ambient conditions. Add the G7 BC to the ballistics calculations.
Odds are it is the scope. But if your target needs "24 up", so be it.
As long as it repeats, it's all good.
Jerry
1. I'll be getting a chronograph asap.
2. My scope (millett LRS 6 - 25) returns to zero repeatedly, so I'm thinking this is indirect evidence of good tracking. I did do a formal tracking test back when I first got the scope, and it tracked well. When I get a chance I'll retest.
3. I suppose the only way to confirm range 100% with this kind of terrain is with a laser rangefinder. I measured carefully with my mildot; I would say error here would be +- 25 yards. I ran calcs on 725 yards worst case, and again the moa is still well past 20 moa required, given 2538 MV. GPS also indicates about 750 yards, but recall the terrain was flat for the first half, then about a 10 degree rise to the target in the 2nd half. There's no way the distance as the crow flies is as little as 700 yards given this, and if so, it would still require about 21.5 moa. Coriolis could take about 2 inches off for this I suppose.
4. calculations with a G7 BC of 0.257 on jbm generates the same result as G1 BC of 0.53: 24 moa with 100 yard zero.
My money is on MV being significantly higher than predicted. It's by far the variable that has the highest impact on drop at a given distance. Specifically, jbm predicts 2750 fps to require 20 moa of adjustment. This is a full 200 fps higher than the value predicted by hodgdon tables for my charge. Charge is measured with rcbs balance beam; 46.9 gr of H414.
2. My scope (millett LRS 6 - 25) returns to zero repeatedly, so I'm thinking this is indirect evidence of good tracking. I did do a formal tracking test back when I first got the scope, and it tracked well. When I get a chance I'll retest.
3. I suppose the only way to confirm range 100% with this kind of terrain is with a laser rangefinder. I measured carefully with my mildot; I would say error here would be +- 25 yards. I ran calcs on 725 yards worst case, and again the moa is still well past 20 moa required, given 2538 MV. GPS also indicates about 750 yards, but recall the terrain was flat for the first half, then about a 10 degree rise to the target in the 2nd half. There's no way the distance as the crow flies is as little as 700 yards given this, and if so, it would still require about 21.5 moa. Coriolis could take about 2 inches off for this I suppose.
4. calculations with a G7 BC of 0.257 on jbm generates the same result as G1 BC of 0.53: 24 moa with 100 yard zero.
My money is on MV being significantly higher than predicted. It's by far the variable that has the highest impact on drop at a given distance. Specifically, jbm predicts 2750 fps to require 20 moa of adjustment. This is a full 200 fps higher than the value predicted by hodgdon tables for my charge. Charge is measured with rcbs balance beam; 46.9 gr of H414.
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
A little too much guesswork in your setup. Best way is to test to see what you actually have.
Odds are it is just the stacking of assumptions and errors that is giving you the incorrect come ups.
Good luck.
If you want to test your scope, I have an article on my website in the Tech section that can help.
Jerry
Odds are it is just the stacking of assumptions and errors that is giving you the incorrect come ups.
Good luck.
If you want to test your scope, I have an article on my website in the Tech section that can help.
Jerry
A chrono, laser rangefinder, and G7 bc are required to rein in some of your variables in order to expect an accurate first round at variable ranges. Have you sighted in at 200 or 300? A 2 round group is also a limited amount of data to make precise adjustments from. Not to be critical, but shooter skill makes a lot more difference at 750 than 100 too.
Google Bryan Litz and buy his book.
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Again, this is essentially useless.
Gunnerlove
CGN Ultra frequent flyer
- Location
- Vancouver Island
Are you 100% sure dialing 24 moa gets you 24 moa?
I'm with jerry here, way too many assumptions.
A scope returning to zero does not mean that 24moa worth of turret clicks results in 24moa of adjustment... It is not uncommon for the click value to be off. To verify this you have to shoot, adjust the scope and shoot at the same position, then measure and calculate the actual adjustment value. Obviously the better you and your gun can shoot, and the farther out you do this test then the more accurate your test will be.
The predicted velocity your using is not a prediction for your rifle, it is the velocity obtained in a test barrel... Not yours!
That reference means absolutely nothing more than a vague estimate.
A chronograph will get you closer to the real velocity but who's to say how accurate that particular chronograph is.
IMO the only real way to verify these things is to get a hand held weather reading apparatus and compare it's reading to a couple known devices. Go to your shooting spot and set up some targets at 50, 100, 200, 400, 600, 800 yards/meters, verify those ranges with 2 or more range finders. Put an aiming point on each, shoot at each one without making any scope adjustments, measure the impact height in reference to point of aim, input the weather, distance, poi/poa, and litz's bc and calculate. You will now have a proven velocity.
A scope returning to zero does not mean that 24moa worth of turret clicks results in 24moa of adjustment... It is not uncommon for the click value to be off. To verify this you have to shoot, adjust the scope and shoot at the same position, then measure and calculate the actual adjustment value. Obviously the better you and your gun can shoot, and the farther out you do this test then the more accurate your test will be.
The predicted velocity your using is not a prediction for your rifle, it is the velocity obtained in a test barrel... Not yours!
That reference means absolutely nothing more than a vague estimate.
A chronograph will get you closer to the real velocity but who's to say how accurate that particular chronograph is.
IMO the only real way to verify these things is to get a hand held weather reading apparatus and compare it's reading to a couple known devices. Go to your shooting spot and set up some targets at 50, 100, 200, 400, 600, 800 yards/meters, verify those ranges with 2 or more range finders. Put an aiming point on each, shoot at each one without making any scope adjustments, measure the impact height in reference to point of aim, input the weather, distance, poi/poa, and litz's bc and calculate. You will now have a proven velocity.
As other have hinted, not all scopes deliver "real" MOA. "Shooting the box" at a known distance would help pin down that variable.
Large target at say 100 yards. Zero optic on centre.
12 "MOA" up+12"MOA" left - shoot a group
24 "MOA" down - shoot a group
24 "MOA" RIGHT - shoot a group
24 "MOA" UP - shoot a group.
Measure distance between perimeter groups, divide by 24, that will give real versus scope "MOA" unit conversion factor
for windage and elevation adjustments.
If you are lucky, conversion factor will be 1.00.
Large target at say 100 yards. Zero optic on centre.
12 "MOA" up+12"MOA" left - shoot a group
24 "MOA" down - shoot a group
24 "MOA" RIGHT - shoot a group
24 "MOA" UP - shoot a group.
Measure distance between perimeter groups, divide by 24, that will give real versus scope "MOA" unit conversion factor
for windage and elevation adjustments.
If you are lucky, conversion factor will be 1.00.
