Thread: Trajectory calculation issue

Check your chrony reading with someones chrony that is calibrated correctly, by placing it in front and behind yours. One should not smudge BC's to get things right the factories spend serious money establishing these figures.

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

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.

A note: I'm slightly drunk

Originally Posted by baldbob

Actually I had a rifle sighted in for 100yd but use meters for ranges soooo maybe if you check shooter settings and set the zero to 110 in the edit rifle data bit or better yet be less confusing and sight in for 100yd.... mine was alll way up the fuck when I had that... the other thing to make sure is right is correct sight height it can make a fair difference to (half bolt dia+half scope dia+distance between scope and bolt)

the difference for me between a 100m and 100y zero is .096moa, ie, too small to correct for

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.