Flatter than calculated trajectory

[17Hornet]

Hi everyone, I'm hoping that someone here is able to help me with a bullet drop related question I have recently come across. I've been a long time lurker here on Accurate Shooter, but finally decided it was time I got an account when I couldn't answer this one. Any thoughts/advice is greatly appreciated.

Note: all ammo/rifle/optics specified at the end of this post for ease of reading. Please forgive the mismatched metric and imperial. Downside of being Canadian. I have tried to provide imperial units for everything to help simplify it.

The issue in brief: we are experiencing substantially less bullet drop than I would expect given a couple of ballistic calculators. We also appear to be achieving a much higher than specified BC (but not high enough to account for the lack of drop).

Preface: I have minimal experience when it comes to long range shooting. My background is as a fairly avid short range target/varmint shooter. All casual, no competition.

I recently helped my friend get into reloading when he bought his new rifle in 22 Creedmoor.

Everything went well at the 100 yard line. We did an OCW test and found a stable load. Then we moved on to seating depth. We were able to find a forgiving seat depth that consistently delivers 1/2"-3/4" groups at 100 yards.

We did a tracking test at 100 yards and found that his scope tracks reasonably well for a hunting optic. The error was around 1/4 MOA over 10MOA travel. I am willing to consider this within margin of error as we only shot 3rd "groups" for this test.

According to JBM ballistics he should be seeing 8.1 MOA of drop at 600 yards with the environmentals below and the calculated BC (8.6 MOA with manufacturer BC). Instead we are seeing 6.25 MOA of drop. Note, the angle to the targets is almost zero, certainly not enough to cause this error (angle compensated distance is just over 590 yards). This lack of drop was replicated on a different day in even less wind.

What is likely to be causing this error? So far, the only things I can see causing this are:

1. I entered something wrong in JBM - please double check this for me
2. There is some sort of updraft present in the coulee (bottom of a rangeland valley) where we are shooting that is causing the bullets not to drop as predicted - we will be retesting in an open field in the future

Thank you for any help/thoughts that you are able to offer,
Tom

If this has been discussed in a previous thread, my apologies. Please post a link to that thread, as I was unable to find it. Also, I will make an amendment to this post once the issue has been resolved.

------------------

Rifle specs:
-Fierce Twisted Rival .22 Creedmoor
-20 MOA rail (unsure of brand)
-Leupold VX-5HD 3-15x44
-2 piece rings (unsure of brand)
-height over bore almost exactly 2"

Ammo:
-CCI 450
-H4831SC
-Lapua 6mm Creedmoor SRP brass
-Hornady 75gr ELD-M

Rest:
-shooting conducted off of sandbags (not touching barrel)

Velocity data on day of shooting:
(I know, small sample size and not the same rounds chronographed for both ranges - I only have 1 chrono)
At 6.3 yards (fps):
3350
3333
3335
3358
3324
Avg: 3340.0, SD: 17.3, ES: 24

At 203 yards (fps):
2998
2985
3010
3016
2984
Avg: 2998.6, SD: 14.4, ES: 32

Calculated G7 BC: .266 (JBM)
Hornady specified G7 BC: .235

Environmentals:
Temperature: 6°C / 42.8⁰F
Elevation: 850m / 2788ft
Wind: approximately 10kph / 6.2mph from SE
Humidity: ~60%
Barometric pressure (corrected) 1022mbar/30.17inhg
Shooting in a coulee (valley) on the prairie
Overcast, mid afternoon

Targets:
-100 yard target is located SSW of bench
-600 yard target is almost due West of the bench

 

[762willdo]

Gremlins……..it always the damn gremlins

 

[Coyotefurharvester]

Interesting from your actual shooting (6.25 moa) target is 500 yards. You need to be 3750 ish to shoot the difference.

 

[17Hornet]

Interesting from your actual shooting (6.25 moa) target is 500 yards. You need to be 3750 ish to shoot the difference.

Yeah, that was what I was seeing as well when I tried to figure out the required speed. He has the correct powder charge - I had him reweigh everything. On a colder day, my chronograph gave 3317fps, and on the warmer day 3340fps.

I checked range with both his rangefinder (Vortex razor) and mine (Nikon 4k). His showed 600 yards on the dot (not compensated). Mine showed 540m (590.5 yards), angle compensated.

 

[Coyotefurharvester]

SFP scope needs to be set to correct power(or did you shoot with a true 100 yard zero) and not using the reticle drop markings or turret adjustment for actual measured drop values.

 

[B Nettesheim]

1 scope is possibly not traveling correctly
2 velocity input is incorrect or not measured accurate
3 bullet bc is off (they do change lot to lot)

 

[Brians356]

There's a huge difference between "drop" and POI below scope LOS. Drop is distance below bore axis (launch angle) and does not relate in an obvious way to what you see on target at 600 yards after zeroing at 100 then dialing in MOA elevation for longer distances. JBM presents a lot of figures which need proper interpretation. When you refer to "drop" it raises an eyebrow is all I'm saying. "Trajectory" is what comes to mind.
-

 

[357Mag]

Hi everyone, I'm hoping that someone here is able to help me with a bullet drop related question I have recently come across. I've been a long time lurker here on Accurate Shooter, but finally decided it was time I got an account when I couldn't answer this one. Any thoughts/advice is greatly appreciated.

Note: all ammo/rifle/optics specified at the end of this post for ease of reading. Please forgive the mismatched metric and imperial. Downside of being Canadian. I have tried to provide imperial units for everything to help simplify it.

The issue in brief: we are experiencing substantially less bullet drop than I would expect given a couple of ballistic calculators. We also appear to be achieving a much higher than specified BC (but not high enough to account for the lack of drop).

Preface: I have minimal experience when it comes to long range shooting. My background is as a fairly avid short range target/varmint shooter. All casual, no competition.

I recently helped my friend get into reloading when he bought his new rifle in 22 Creedmoor.

Everything went well at the 100 yard line. We did an OCW test and found a stable load. Then we moved on to seating depth. We were able to find a forgiving seat depth that consistently delivers 1/2"-3/4" groups at 100 yards.

We did a tracking test at 100 yards and found that his scope tracks reasonably well for a hunting optic. The error was around 1/4 MOA over 10MOA travel. I am willing to consider this within margin of error as we only shot 3rd "groups" for this test.

According to JBM ballistics he should be seeing 8.1 MOA of drop at 600 yards with the environmentals below and the calculated BC (8.6 MOA with manufacturer BC). Instead we are seeing 6.25 MOA of drop. Note, the angle to the targets is almost zero, certainly not enough to cause this error (angle compensated distance is just over 590 yards). This lack of drop was replicated on a different day in even less wind.

What is likely to be causing this error? So far, the only things I can see causing this are:

1. I entered something wrong in JBM - please double check this for me
2. There is some sort of updraft present in the coulee (bottom of a rangeland valley) where we are shooting that is causing the bullets not to drop as predicted - we will be retesting in an open field in the future

Thank you for any help/thoughts that you are able to offer,
Tom

If this has been discussed in a previous thread, my apologies. Please post a link to that thread, as I was unable to find it. Also, I will make an amendment to this post once the issue has been resolved.

------------------

Rifle specs:
-Fierce Twisted Rival .22 Creedmoor
-20 MOA rail (unsure of brand)
-Leupold VX-5HD 3-15x44
-2 piece rings (unsure of brand)
-height over bore almost exactly 2"

Ammo:
-CCI 450
-H4831SC
-Lapua 6mm Creedmoor SRP brass
-Hornady 75gr ELD-M

Rest:
-shooting conducted off of sandbags (not touching barrel)

Velocity data on day of shooting:
(I know, small sample size and not the same rounds chronographed for both ranges - I only have 1 chrono)
At 6.3 yards (fps):
3350
3333
3335
3358
3324
Avg: 3340.0, SD: 17.3, ES: 24

At 203 yards (fps):
2998
2985
3010
3016
2984
Avg: 2998.6, SD: 14.4, ES: 32

Calculated G7 BC: .266 (JBM)
Hornady specified G7 BC: .235

Environmentals:
Temperature: 6°C / 42.8⁰F
Elevation: 850m / 2788ft
Wind: approximately 10kph / 6.2mph from SE
Humidity: ~60%
Barometric pressure (corrected) 1022mbar/30.17inhg
Shooting in a coulee (valley) on the prairie
Overcast, mid afternoon

Targets:
-100 yard target is located SSW of bench
-600 yard target is almost due West of the bench

17Hornet -

Howdy !

For your trajectory calculations, have you input the true height of the scope above the bore centerline ?


With regards,
357Mag

 

[17Hornet]

SFP scope needs to be set to correct power(or did you shoot with a true 100 yard zero) and not using the reticle drop markings or turret adjustment for actual measured drop values.

We were dialing for all shots beyond 100 yards. When dialing in based off of the chart I produced with JBM, he was shooting well over the target.

 

[17Hornet]

17Hornet -

Howdy !

For your trajectory calculations, have you input the true height of the scope above the bore centerline ?


With regards,
357Mag

Hi 357Mag,

I did input the measured distance between bore centreline and optic centreline. It was almost exactly 2.0".
Thanks

 

[mongo chicago]

These ballistic calculators are GENERALLY for "getting on paper". They are not precise although sometime you can get real close if you are lucky.

 

[17Hornet]

There's a huge difference between "drop" and POI below scope LOS. Drop is distance below bore axis (launch angle) and does not relate in an obvious way to what you see on target at 600 yards after zeroing at 100 then dialing in MOA elevation for longer distances. JBM presents a lot of figures which need proper interpretation. When you refer to "drop" it raises an eyebrow is all I'm saying. "Trajectory" is what comes to mind.
-

I accept your terminology correction, but I don't see how I could be misinterpreting the output chart that JBM gave me. In the drop column, it says 8.1 MOA. Am I incorrect in assuming this can be used as a dialup correction?

 

[17Hornet]

1 scope is possibly not traveling correctly
2 velocity input is incorrect or not measured accurate
3 bullet bc is off (they do change lot to lot)

1. We had tested this on the day. As you can see from the original post, tracking was pretty close. Within .25MOA over 10MOA of travel. Not perfect, but certainly not off by 1.9MOA.
2. This is possible, but I only have one chronograph to check it with. Velocity data is within the expected range given the powder charge, and it has been accurate with other rifles previously. To be fair, it is a crappy old Caldwell unit. As was previously pointed out, a MV of about 300fps higher than measured would be required.
3. We also tested this when we were out. Calculated BC came out to .266 G7, instead of Hornady's spec of .235 G7. Substantially higher. With the corrected BC, predicted drop at 600 yard comes to 8.1MOA from 8.6MOA. Still a fair ways off of the observed 6.25MOA correction.

Thanks for the suggestions!

 

[17Hornet]

These ballistic calculators are GENERALLY for "getting on paper". They are not precise although sometime you can get real close if you are lucky.

I can appreciate that they are not going to be perfect, but both the Hornady 4DOF calculator and JBM were off by nearly 2MOA. Is this typical? It just seems like a lot...

 

[Brians356]

I accept your terminology correction, but I don't see how I could be misinterpreting the output chart that JBM gave me. In the drop column, it says 8.1 MOA. Am I incorrect in assuming this can be used as a dialup correction?

I wish I could see the output page from your JBM run, with all the reported fields. Normally "bullet drop" is in inches, and is how far the bullet would have dropped below line of departure if fired horizontally when it reached the terminal distance. In a vacuum, the drop would equal how far the bullet would fall vertically if dropped from the muzzle, in the given time of flight.
-

 

[Brians356]

I can appreciate that they are not going to be perfect, but both the Hornady 4DOF calculator and JBM were off by nearly 2MOA. Is this typical? It just seems like a lot...

That's a lot, and that's why I suspect there is confusion about terminology.
-

 

[Brians356]

I just ran a JBM test, and see it uses "Drop" the way most calculations use "Trajectory", so forget all my comments. "Bullet drop" as I described is always algebraically positive, and starts at 0.0 and increases always. "Trajectory" is relative to line of sight (LOS), starts negative (below LOS), rises through 0 at LOS, rises to mid-range height, starts falling to LOS, goes negative below LOS, etc. JBM calls it "Drop" which I feel is a poor choice.
-

 

[big5ifty]

You seem to be using the calculator with the correct values and options set.

JBM is my preferred solver, I go the the range with a printout with an extra empty column for actual dialed, and I record the actual dialed for an impact at a given range. I decided a while back that it's the Vortex Fury AB,or paper, but that's another story.

It is normal to have to massage the numbers to fit actual with any calculator or ballistic solver.

There is a simple way to approximate the calculated curve to the actual. Set up three targets at 100, 200 and 300.

Zero on 100, then without dialing the scope, hold center on the 200 and 300, 3 shots each.

Find and measure from the center of the target to center of the groups on the 200 and 300, that is your bullet drop at those ranges.

Remember to exactly measure each target range, the velocity, and the distance to the chrony.

This assumes and requires that you can produce at least a sub-moa group at 300, otherwise the scatter may skew the result.

Mathematically, you are finding find the equation for a parabolic curve using three known points.

Then adjust the parameters in JBM until the output chart matches up with what you have recorded as actual.

I'd be interested to know your outcome if you do try this.

 

[17Hornet]

I just ran a JBM test, and see it uses "Drop" the way most calculations use "Trajectory", so forget all my comments. "Bullet drop" as I described is always algebraically positive, and starts at 0.0 and increases always. "Trajectory" is relative to line of sight (LOS), starts negative (below LOS), rises through 0 at LOS, rises to mid-range height, starts falling to LOS, goes negative below LOS, etc. JBM calls it "Drop" which I feel is a poor choice.
-

Yeah, that's where I was getting my terminology from. I agree that having a separate term for the angular "drop" makes more sense.

 

[17Hornet]

You seem to be using the calculator with the correct values and options set.

JBM is my preferred solver, I go the the range with a printout with an extra empty column for actual dialed, and I record the actual dialed for an impact at a given range. I decided a while back that it's the Vortex Fury AB,or paper, but that's another story.

It is normal to have to massage the numbers to fit actual with any calculator or ballistic solver.

There is a simple way to approximate the calculated curve to the actual. Set up three targets at 100, 200 and 300.

Zero on 100, then without dialing the scope, hold center on the 200 and 300, 3 shots each.

Find and measure from the center of the target to center of the groups on the 200 and 300, that is your bullet drop at those ranges.

Remember to exactly measure each target range, the velocity, and the distance to the chrony.

This assumes and requires that you can produce at least a sub-moa group at 300, otherwise the scatter may skew the result.

Mathematically, you are finding find the equation for a parabolic curve using three known points.

Then adjust the parameters in JBM until the output chart matches up with what you have recorded as actual.

I'd be interested to know your outcome if you do try this.

Leaving the second column blank for actual recorded values is brilliant. We shall have to give that a try.

Hopefully I'll be able to get out with him next weekend and run some more tests. With how wet the ground is, we'll be out shooting in a field, so I'll see if my updraft theory holds any water.

Either way, I'll report back here once I have some more data.


Thank you to everyone who has chimed in with ideas and suggestions!