Thread: Trajectory calculation issue

Originally Posted by horihunter

scope height is 1.92"
pressure 986 hpa
I have reduced speed to 2890fps and can get it within .5moa of the real world figures now which may just be as close as I can get it. It ranges from .1 to .5 MOA low across the given ranges leaving a spread of .4, allowing for shooter error I might just add one click to the zero and should have it pretty close.
off to the range again tomorrow...

Had a quick play, I have dropped it to 2910 fps and tweaked the BC a bit and got it mostly to .25 MOA but I think the range at 568yards may be a bit suspect...

Originally Posted by horihunter

While at the long range shoot in Kurow we had the opportunity to confirm our drop charts and collect some real data. I got the following real world results (all in yards because 1000 yards sounds way cooler than 900m) rifle is zeroed at 100m (110 yards)
343 - 4moa
480 - 8.5 moa
568 - 11.25 moa
697 - 15 moa
850 - 20 moa
5 degree angle
7mm mag firing 168gr berger vlds at 2960 fps (thats what chrony told me but 2900 seems to replicate the trajectory better)
I am using shooter and having issues getting the numbers to align to the real world results. Anyone want to join me in trying to get things to line up?
The frustrating thing is my other 2 rifles are spot on straight from shooter so think something must be wrong with the input data.
Might have to go and do another long range dady and do the data collection again to confirm but it was all pretty convincing on the day with shots landing as they should with the various corrections etc.
Wondering if maybe when we were ranging the steel targets we were actually getting readings off the ground in front/behind
Gimp did you measure the ranges to each plate from the plates at all? Did anybody else get different ranges for the plates?

Very interesting you should post this, as i have just been through the same process myself!

I struggled a bit to get my actual drops to match the numbers exactly, I played with 4 different ballistic programs, trajectory validation, BC, FPS, and even steeped BC's.

Lernt heaps about fudging charts and got heaps of advice from people that know their shit, and in the end i realised that what you are trying to achieve with a drop chart is to create numbers that match your ballistic curve as close as possible. You are talking about EXACT points at an EXACT distance, which may be a bit to much to ask for. Should however be possible to match most of them with a singe click out here and there.

Some important things to consider that will effect your numbers, especially at the range you are working to: You have to measure your EXACT scope height above bore, you have to make sure your scope clicks are exactly .25 MOA (if they are in MOA), you have to make sure you are not getting any backlash in your scope while shooting those groups - a good tip is to wind your scope up and down between each shot at the longer ranges, make sure your elevation and air pressure match your sighting in conditions.

I can have a play tonight with the numbers, need elevation, pressure, temp, scope height. Is your Zero EXACT in the bull or is it +- 1/2

As per another thread, this is also one of the reasons where it helps being able to shoot a small group at the longer ranges!

Any chance the scope is not dialing right?

What time are you going out to the range?

So 986 was the absolute press for the altitude we were at? That would make it about 730ft above sea level, you've applied that correctly

This is what I did if it helps..

BC 0.550
2920fps
730 ft
29 hg
Scope Height 1.8

340 - 4.25
480 - 8
568 (odd one) - 10.5 (sure that was not 590ish?)
700 - 14.75
850 - 20.25

I'm wodering if it should be 590is I see KG and I both had 564-568 while outdoorlad had 602... that would certainly help explain that one. Will try something at the same range again in weekend if all goes well

How exactly is someone elses chrony 'calibrated correctly'
By firing a projectile of known velocity over it? then how is the 'known' velocity 'known'?? by being shot over a calibrated chrony?? (repeat the process)

Factory BCs are at XXX speed bracket.
If you are either side of that exact speed it was tested, the the BC is invalid as a given finite number - BCs are a curve - the number the factories supply is an average over a speed range and only fits that bracket even roughly. You can be either end of the bracket and the BC is different.
Same with G7, run it faster or slower than they dont fit.
These are facts not opinions and are almost 100 years old

Personaly id be more inclined to tweak the BC 1st before playing with the FPS (which may be a few FPS out but is still a known factor)

In this case HH has varified his drop at the longer ranges he will be shooting at which helps set the rest of the chart. (as long as he confirms that mid-range drop )

Will be pretty hard if not impossible to match exactly, but tweaking the BC a bit can match the curve to .25 MOA, which interestingly enough is .25 UP and .25 DOWN some ranges, so the ballistic curve is running smack in the middle between what he shot on the forum range shoot.

I wonder where that info came from.

Originally Posted by Terminator

I wonder where that info came from.

Easy daisy

A note: I'm slightly drunk

I don't remember the exact ranges from the shoot and I did not log anything because I have to change my load since it decided to start piercing primers after sitting in the sun all day.

This is slightly unrelated, but re: to measuring BC yourself from drops etc.

From Brian Litz' book:

175gr 7mm Sierra Matchking (random projectile selected) G1 BC measured with doppler radar

1500fps: .584
2000fps: .641 (.063 up from 500fps difference)
2500fps: .656 (.015 up from a 500fps difference)
3000fps: .677 (.021 up from 500fps difference)

Average over bullet flight from 1500-3000fps: .639

Points:
-I see no reason why the trend of "little variation for 500fps speed increase at higher speeds" wouldn't continue to be a similar negligible difference at 3000/3500fps.

-You will never be able to measure a difference in the real world from that sort of difference in BC, considering all the other factors and the error factor of the instruments you are using. A .1 difference in G1 BC gives you something like 2" different in drop at 500 meters. I challenge anyone who claims that their zero, data, shooting, scope tracking is perfect to the degree that they can accurately measure that sort of difference

-If you do have a higher initial BC, it will drop closer to the average anyway at the ranges where it becomes more relevant



I'm not saying anything about logging your real world observed results and trying to make them fit a ballistic calculation for the purposes of hitting stuff. Just saying that there are limits to the accuracy you an ascribe to field gathered data.

If you have a faulty chronograph, bad zero, scope that doesn't track correctly, bad range, bad shot, bad data input into calculator, bad data from weather meter, then your results from program may not line up properly. So log everything and simply work from your notes instead for data.