Was out with a mate over the weekend trying to adjust his drop chart.
270win, 140gn berger, 3115fps, 74F,43%hum,800feet alt,29.8 pressure.According to FTE we should have been 32" low @ 525yds, we were in facf 39" low, so a discrepency of 7".
We adjusted the fps up to give us the right drop in FTE but then of course we were out at the closer ranges and had to revert back to the original data.
Should we of altered the BC instead?
We realy haven't had a great deal of success with these Ballistics programs, or are we missing some vital data that we are not putting in.
what are your experiences using these programs??


regards
Griff

At what range were you zeroed?
What program were you using?

With a 200 yard zero, JBM says you should be 39.3" low at 525 yards.

quote:

what are your experiences using these programs??

They are great and indispensible!!

The output is only as good as the input you feed it. The most accurate atmospheric data you can feed it is derived from a Kestrel with the altitude set to -0-. Actual station pressure not adjusted for altitude. In your program set altitude to -0- as well. You may need to read up on your program.

If that is not possible, use the atmospheric conditions gotten from the nearest weather station.

As far as the other inputs of velocity, BC, bullet weight, sight height (important) etc...you'll have to most likely alter what you THINK they are while putting them into the program and keep playing with them till the program's output matches what you see for actual drop.

I use BulletFlight for Apple products. JBM makes a good one, "Shooter" has been written about by Brian Litz and "Field Firing Solutions" is outstanding to run on a PDA. FFS is probably the best but costs about 150$ without the hardware to run it on.

AS,
zero range was 200yds, using FTE.

regards#
griff

If everything else was correct, (velocity, sight in range, atmospheric conditons, etc.) then I would adjust the bc until it matched your real life trajectory numbers.

Good for you for double checking the info with real life shooting.

Well, so JBM gave the right result.
Looks like it's either an issue with FTE or your inputs?

PS. Can't say I'm familiar with FTE?

For long range calculations the exact height of the center of the scope above the center of the bore is also critical. An incorrect input changes the angle of departure. The error of which increases as the range increases. Assuming you also used the factory supplied BC of the bullet.

Depending on twist and actual velocity that BC may also not be correct. Remember that a BC measures the efficiency a bullet moves through air. The higher the BC of a given bullet at a given velocity demonstrates a higher efficiency and that is then a measure of it's stability. I've been measuring BCs in various twist .223s and .308Ws. I find it consistent that each bullet will give a higher BC in a twist it is actually compatable with. For example; .308W with 10, 11, 12 and 14" twists with the same load using the 175 gr MK. In the 10" and 14" twist the BCs hover around .48. In the 11" twist they are right at .5 and in the 12" twist they are around .52 - .53. This is with the same load at pretty close to the same velocities. Obviously, even with comparable velocity, the 175 MK from the 12" barrel will shoot appreciably flatter at long range.

If you've put in all the correct data into a good program and know the zero range and a an actual drop at a longer range you can adjust the BC until the computorized drop matches the actual drop at the longer range. That will give you the correct BC for that bullet from your rifle. Using that corrected BC the other trajectory corrections will then be very close to correct....if you are using a good reliable program.

Larry Gibson

I should probably mention I also used the Litz corrected G7 BC for that bullet in my calculations.

quote:

Originally posted by Antelope Sniper:
I should probably mention I also used the Litz corrected G7 BC for that bullet in my calculations.

+1

the G7 profile is the one to use Griff, the G1 profile is not accurate for VLD designs and would need adjusting downrange for the difference.

What's happening is that the initial G1 BC is higher at higher velocity than lower.
This is consistent with your findings as you get more drop than you expect at longer range and if you correct you get a higher mid trajectory point because the bullet is travelling slower than your G1 model predicts at 525.

For example Sierra list several B.C.s for their bullets depending on velocity:

.270 135gr SMK:

.488 @ 2800 fps and above
.482 between 2800 and 2000 fps
.440 between 2000 and 1500 fps
.390 @ 1500 fps and below

Berger publish the G7 BC values and as AS says, should give you a much more accurate set of data.

The online JBM ballistics is rather good and Jon will pleased to know you get an app for the iphone for it!

Link to online JBM calculator

Ghubert,
thanks for that, I would of thought that when you selected a bullet from the drop down list that it would select the correct BC, that is something I will have to check!


Larry,
that is something I have never ever considered,how do you check BC in your given rifle?

regards
griff

Sorry guys!
Made a complete arse of the first thread.
I got it the wrong way round, the actual drop was 32" and FTE told us it would be 39".
That is why we altered the velocity to give us 32"@525yds.

regards
griff

Griff, how did you derive your velocity. From a book, or did you chronograph this specific load, in this specific gun?

Chrono's don't work perfectly all of the time. After you put in the best information about the load and the atmosphere you have, you will still have to tweak the velocity or BC input (most likely. I have hit it perfectly before)

quote:

Originally posted by griff:
Larry,
that is something I have never ever considered,how do you check BC in your given rifle?

regards
griff

I actually measure them using an Oehler M43. It uses muzzle screens (I usually set the start screen 15' from the muzzle) and another set of screens in front of the 100 yard target. That measures time of flight and then converts to BC, same way all the ballistic labs do it. It also corrects the muzzle screen velocity to muzzle velocity plus giving lot of other information including psi via a strain gauge on the barrel over the chamber.

Larry Gibson

Hi Griff, we need some more information mate.

1. How did you derive the velocity given above as 3115 fps? Quickload, Chrono, loadbook or estimate from ballistics?

2. "Zero range was 200 yards, using FTE". Does that mean you physically zeroed the rifle at 200 or that you're predicting the zero range based on the program's estimate, based on the drop at 525?

3. Do you have any of the intermediate drops, the one that you said were out when you corrected for the new velocity?

Once again I tend to agree to agree with AS, it sounds like a velocity error.