Ballistic question - I am stumped

[Arkansasdad]

Playing with 150 PVA Cayuga in my factory tikka 7mm mag.
Claimed BC .311 G7. Cody Adam's trued his to .300ish G7

Velocity 3150 (lab radar and magnetospeed confirmed.)
Sight in 2moa high at 100, confirmed zero at 255 yards.
Elevation 3365, pressure 26.45, humidity 50, temp 65. (Kestral verified and also consistent with previous shooting at this location.
Predicted drops (shooter and hornady)
400 - 2.75 moa
600 - 7.25 moa
1k - 18.75 moa

Actual drops
400 - 3.25 moa
600 - 9.25 moa
1k - 20.75

To get drops to match at 600 I have to drop the DC or velocity ridiculous amounts, which predictably puts it way off at 1k. Checked tracking on the scope and it's spot on. After checking tracking, I shot again at 100, 255, 400, and 600 with the same results.

Any ideas would be appreciated and I am out of them.

Never believe the listed B.C. from a manufacturer, I cannot remember but one that was on track with the listed B.C. The way B. C. is found by these companies is by data math and not proofing. Looks like you have your own data now, extrapolate your data to find a real B.C. on these bullets and use that from now on. Happy shooting.

 

[FEENIX]

you know I don't know if they're really "stretching" the truth tho. It's math. They "could" give multiple BC's based on several velocity ranges, like Sierra and Hornady does, or they could give an average BC of the projectile over those velocity ranges, but the BC is a velocity based due to the drag coefficient of that projectile. Seems That they just give the BC based on the higher muzzle velocity at a specific DA. Putting down the BC for the highest velocity isn't necessarily un true

Agreed! As per your reference in #21, they clearly noted how they derived with their BC.

"If" I use Berger's ballistic calculator with the OP's actual information, they are pretty much in line with the drop calculated projections.

 

[StanleyActual]

Agreed! As per your reference in #21, they clearly noted how they derived with their BC.

"If" I use Berger's ballistic calculator with the OP's actual information, they are pretty much in line with the drop calculated projections.

View attachment 295888

yeah that's pretty well dead on.

 

[M Jager]

For those asking my scope height is 1.8

As for pressure in my area being low, it's always low in this area. I just walked out my door and took some readings.
Temp 71.4
Humidity 30
Alt 3700
Density Alt 5574
Pressure inHg 26.13

 

[Doom2]

Your pressure is Ok. Make sure your Calculators are using station pressure and not corrected pressure and altitude. I ran your numbers in JBM and they were close to your Dope

 

[M Jager]

Doesn't matter if you have a problem. It's a fact of shooting. The OP is shooting at 600 and 1k. PBR is not the issue, plus it can still be done at 100y. However that's beside the point.


proof:
You'll notice that the numbers match the OP's issue at 600y.View attachment 295856View attachment 295857

Yet another case of garbage in, and garbage out. Zero matters. It might be just a number, but you are literally dealing with just numbers in this game.

Unless I am missing something drastic, your example shows that the zero range isn't the issue. My rifle is right at 2 moa high at 100. If I zero at 100, (drop my zero 2 moa), I would now need 11.25 moa to hit 600 so still roughly 2 moa more predicted and we are back to making extreme cuts to velocity or bc inputs to force it line up.

 

[M Jager]

What other bullets have you tried and how did

What other bullets have you tried and how did they work?

I haven't tried anything else in this gun. I could run some 162 elds just to check. In other guns, various hornady and sierra bullets match up reasonably well.

 

[M Jager]

Never believe the listed B.C. from a manufacturer, I cannot remember but one that was on track with the listed B.C. The way B. C. is found by these companies is by data math and not proofing. Looks like you have your own data now, extrapolate your data to find a real B.C. on these bullets and use that from now on. Happy shooting.

Naturally, we don't trust the box. My question is more of why am I so drastically off from both the box and what others are observing. Furthermore if I true my bc based on my 600 yard results, I am way off at 1k.

 

[Blackdirt Cowboy]

Have you tried truing your velocity to 600 yards and then truing bc to 1000 yards? What does that look like?

 

[M Jager]

Main reason you zero 100 is that the effects or weather aka atmospherics are negligible at that range. The further out it you go the more theyre being included into your rifles "true" zero.

Anyways. Back to the OP, in order for me to match your confirmed hits with the data you gave at 400 and 600, I have to drop your MV to 2775 FPS with a G7 of .311. And then run your G7 BC from .311 to .4 to get to your confirmed 1k dope of 20.75.

What optic are you using? And have you checked your tracking?

View attachment 295862

Zeiss v4. As stated in the OP, I checked tracking between trips to shoot at 600 as it's the first thing I thought of

 

[Mikecr]

As per your reference in #21, they clearly noted how they derived with their BC.

No they did not.
Their only clue provided is MV.
They did not state their air density conditions, which is huge to BC at any velocity.
G7 BC for example is based on bullet drag (affected by air density) then adjusted by drag coefficients looked up on the G7 table by MACH #(again dependent on air density). NOTHING about BC can be credibly referenced without air density information.

They use Mach # because supersonic aerodynamics are not directly dependent on velocity. It's really more about shock waves (each MACH). It's not until subsonic that velocity becomes a direct contributor.
Many BCs posted do not provide velocity info, some do and at different velocities. When there is a lot of shifting of BC per velocity, it means the G curve applied is not a good match to the aerodynamics of the bullet. If the curve matches well, BC will hold at each velocity. It's for that driving folks to G7. Berger uses an average BC to normalize minor G curve departures. Good idea there

 

[Mikecr]

I see the sentiment here that posted BCs are usually a lie, but that might not be the case at all.
When you consider BC between different bullets, take the time to find out what atmospheric conditions the BCs are based on.
This information needs to be carefully loaded as a baseline into your ballistic software.
Then, you would enter field conditions into that software, and then it can properly adjust for local BC.

If you are not doing this, but instead lying to the software, then it is you lying -not the bullet maker.
So it is at least important to figure out what is going on. The REAL cause of an issue.

 

[Maol]

As we know the BC changes the farther you get from the muzzle. Some bullet makers adjust for for it some don't. Hornady I believe does for their bullets in their app. How fast the bullet goes to sleep is a factor as well. Different twists, barrel length and rifling matter. Monos are more sensitve. Still a very surprising delta.

 

[FEENIX]

No they did not.
Their only clue provided is MV.
They did not state their air density conditions, which is huge to BC at any velocity.
G7 BC for example is based on bullet drag (affected by air density) then adjusted by drag coefficients looked up on the G7 table by MACH #(again dependent on air density). NOTHING about BC can be credibly referenced without air density information.

They use Mach # because supersonic aerodynamics are not directly dependent on velocity. It's really more about shock waves (each MACH). It's not until subsonic that velocity becomes a direct contributor.
Many BCs posted do not provide velocity info, some do and at different velocities. When there is a lot of shifting of BC per velocity, it means the G curve applied is not a good match to the aerodynamics of the bullet. If the curve matches well, BC will hold at each velocity. It's for that driving folks to G7. Berger uses an average BC to normalize minor G curve departures. Good idea there

It might not be the information you wanted to see, but they did provide how they derived their numbers, which is more information than most bullet manufacturers' claim of their BC.