Adjusting bc or velocity to match drop

[Joefrazell]

Bergers twist calculator says your stabile at 1.57 Sg at 2400 ft elevation. Something else is wrong, maybe your getting extreme muzzle exit pressure, upsetting the bullet out of the gate.

I'm not really sure. I know I'm not pushing it very hard though. 2800 out of a 26" with h4831sc. I'd imagine the magnettospeed is accurate that's why I suspected the bc.

 

[gohring3006]

I'm not really sure. I know I'm not pushing it very hard though. 2800 out of a 26" with h4831sc. I'd imagine the magnettospeed is accurate that's why I suspected the bc.

Exit pressure is usually due to a certain powder not agreeing with your your bullet/barrel it's often overlooked when people have a very accurate load at short range but it falls apart the farther you go out. It can also affect gyroscopic stability and increase drag, reducing BC.

 

[cjuve]

What are you using for your atmospheric inputs and what type of calculator are you using ? Also, what range finder are you using? Does it do LOS or is it giving true horizontal distance?

 

[Joefrazell]

Temp is from my pickup and everything else from the local weather reports. I havent got a kestrel yet. using sterlok but not the pro version. I'm using the new nikon 4k so yes it does angle compensation.

We're talking almost two minutes difference of what it would be at .696.

The thing is that it worked at 705 as well because I was shooting low at .696 by about .5 moa

Do most need to mess with the bc like this or is this definitely something else completely?

 

[Joefrazell]

I think I may have messed up the temp by about 5 degrees. I think that's some of the issue. If I put it in as 70 instead of 75 and .670 as the bc its lining up just fine. Its crazy how precise everythings gotta be. It's fun

 

[cjuve]

Temp is from my pickup and everything else from the local weather reports. I havent got a kestrel yet. using sterlok but not the pro version. I'm using the new nikon 4k so yes it does angle compensation.

We're talking almost two minutes difference of what it would be at .696.

The thing is that it worked at 705 as well because I was shooting low at .696 by about .5 moa

Do most need to mess with the bc like this or is this definitely something else completely?

I am not at all familiar with the Sterlok so I can't really help you there but inputting your velocity with the .696 BC with fairly standard atmospherics 70 F
2400' el
30.1
65 RH
My chart shows 32 MOA
If you were to accidentally input station pressure of 27.4 into your calculator which would be pretty close for 2400'
It would generate an error of 1.5 MOA.

Generally speaking I have not had to adjust BC when using velocities from my magnetospeed and berger bullets as long as the bullet is stable in bergers stability calculator.

 

[tmmcampbell]

I had the same issue one time. Got it all dialed in by decreasing velocity. The next range trip discovered my zero was one minute low. Taught me to check my zero first.

 

[Greyfox]

Your BC adjustment seems high and indicates there may be something else going on. The Bergers are generally quite close, as is the velocity reading from the Magnetospeed. Any BC adjustments I have made with the Bergers have been at most, 20% of yours. The few times that ai have seen this issue, ruling out equipment, rifle support,etc, it turned out to be either an incorrect zero, or incorrect atmospheric conditions. At 100/200 yard zero's, parallax setting can get tricky. I double check my settings at 100 and 200 yards...and make sure they are "precise". Sometimes the local reports for pressure work fine, most times the do not match my Kestrel or my Ballistic RF. Just some thoughts.

 

[Joefrazell]

Thank you all for the help. I think I need a kestrel lol. Im pretty sure my inputs caused it

 

[tmmcampbell]

Everyone needs a kestrel

 

[J E Custom]

If you use a ballistic range finder, It is best to adjust the BC of the bullet. This will be the most consistent throughout the full range of your shots. Most bullet makers use Ideal conditions to arrive at the best BC for their bullets.

If you adjust the BC in your environment It is not uncommon for it to be different than advertised. At sea level, none are correct so we have to adjust BCs.

Shoot as many different distances as possible, and adjust the BC to something that will closely match you trajectory and it will also solve the problem of different BCs at different velocities at distance.

Just My opinion

J E CUSTOM

 

[105Coues]

My chart shows 32 MOA
If you were to accidentally input station pressure of 27.4 into your calculator which would be pretty close for 2400'
It would generate an error of 1.5 MOA.

Not to hijack the thread but if I am using station pressure and a zero elevation. Are you saying there would be an error as opposed to using elevation? I'm asking because I think I had to adjust my bc to make it right at 1k.

 

[Brant]

Beating a dead horse here. A lot of good info in this thread already. without a kestrel for a live barometric pressure and temp reading you could easily have a 2 minute shift at 1150. Also make sure if you are putting the actual barometric pressure into your calculator you have the option to set the pressure to "absolute pressure". This means that your elevation is irrelevant because you are actually measuring the uncorrected barometric pressure and not a station pressure which is corrected to correlate with sea level pressure. This will actually make you impact high for a given distance though so probably not the issue here. So don't stress too much until you are shooting with a kestrel beside you. Also make sure if you can that you check your velocity on the same day that you are shooting long range because ammo temp can make a big difference depending on powder primarily in your velocity. If you got velocity data in hot weather and your ammo was hot temperature wise and you went out to distance with colder ammo this could be a factor that easily accounts for 2 moa at 1150. Just something to keep in mind and check your velocity in a multitude of different ammo temps to see how sensitive your particular load is.

 

[cjuve]

Not to hijack the thread but if I am using station pressure and a zero elevation. Are you saying there would be an error as opposed to using elevation? I'm asking because I think I had to adjust my bc to make it right at 1k.

No, that is not what I am saying. I am saying if you are incorrectly inputting a station pressure into a field that should be barometric pressure with a corresponding elevation with cause an error.