Flatter than calculated trajectory

[Eagle Six]

Also, what is your zero height and zero offset. Another posted mentioned that and it is important, so I entered zero for both but if you can measure that I would appreciate it. Measure how many inches (decimals) your POI is off from your POA both elevation and windage either left/right and up/down.

Zero Height 0 inches
Zero Offset 0 inches

 

[dcali]

JBM is good. There is virtually no difference between JBM and AB under the covers.

 

[Eagle Six]

When I enter your data in the JBM, I get very close to what you get. I'm at a loss as to why they are off so much. There always seems to be a difference between calculators depending on how I hold my tongue and what phase the moon is in, but I haven't seen this much difference before.

 

[Eagle Six]

Gremlins……..it always the damn gremlins

If we keep poking at it, we will find the answer, but so far I think @762willdo is correct!!!

 

[17Hornet]

Also, what is your zero height and zero offset. Another posted mentioned that and it is important, so I entered zero for both but if you can measure that I would appreciate it. Measure how many inches (decimals) your POI is off from your POA both elevation and windage either left/right and up/down.

Zero Height 0 inches
Zero Offset 0 inches

Unfortunately I don't have his targets here with me, so I'll have to go off memory. I'll see if he still has them to measure.

We had zeroed for his cold bore shot, so the majority of the group is high.

Zero height: probably about 3/8" above POA

Zero offset: 0

 

[17Hornet]

When I enter your data in the JBM, I get very close to what you get. I'm at a loss as to why they are off so much. There always seems to be a difference between calculators depending on how I hold my tongue and what phase the moon is in, but I haven't seen this much difference before.

Thank you for running it in JBM! Thought I was losing my mind. Lol

 

[Webster]

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

I would think the only thing that could account for it is the published BC was not determined accurately. Adjust the BC until the program matches your real data. I have seen on this website where a serious competitor complained to a bullet manufacturer about the BC. The manufacturer retested and published a new BC. It probably cost the bullet makers a lot of money to test a large number of bullets.

As Eagle Six said it may be the program itself. Try the free Berger ballistics program on their website.

 

[Jeff Porter]

I don't know about your range restrictions with pyrotechnics but a smoke canister in the coulee while shooting would show a big up draft while you were shooting.

 

[Eagle Six]

Zero height: probably about 3/8" above POA

Zero offset: 0

That takes care of most of it. AB is now showing 8.0 MOA up

Edit: This is incorrect, I have made another post later on to explain.

 

[17Hornet]

I would think the only thing that could account for it is the published BC was not determined accurately. Adjust the BC until the program matches your real data. I have seen on this website where a serious competitor complained to a bullet manufacturer about the BC. The manufacturer retested and published a new BC. It probably cost the bullet makers a lot of money to test a large number of bullets.

As Eagle Six said it may be the program itself. Try the free Berger ballistics program on their website.

I had tried that, but to get to 6.25 MOA at 600 yards, would take a G7 BC of 1.3, which I can't believe is correct.

 

[17Hornet]

I don't know about your range restrictions with pyrotechnics but a smoke canister in the coulee while shooting would show a big up draft while you were shooting.

That's a good idea. Unfortunately we won't be able to get back into the coulee for quite a while due to mud. Next test will be in a flat field (with a hill at the end).

 

[17Hornet]

That takes care of most of it. AB is now showing 8.0 MOA up

Hi Eagle, I'm a little bit confused here. Wouldn't having a point of impact above point of aim result in a reduced dialup at range? Your initial value was 5.75 MOA iirc.

Observed drop from zero is 6.25 MOA, so maybe 6.75MOA from zero if it was zeroed POI/POA.

JBM shows 8.1 MOA with zero/zero and 7.7 MOA with the zero height taken into consideration.

Certainly a factor, but I'm wondering if perhaps there is some issue with the way JBM handles altitude? At this point it's about the only thing I can think of happening in the software.

 

[Brians356]

The reason we are setup this way is due to the topography of the coulee - this is the way the backstops have naturally arranged themselves.

Yeah, I noticed the reference to coulee, and as others pointed out, vertical air motion is not unlikey. I grew up in canyon country.
-

 

[17Hornet]

Yeah, I noticed the reference to coulee, and as others pointed out, vertical air motion is not unlikey. I grew up in canyon country.
-

Perhaps that, combined with what a few others have suggested has compounded to make affect the actual trajectory. I guess we will see if some of it goes away when shooting in a field. Thanks!

 

[Brians356]

Incidentally, when shooting in terrain, it helps to factor in even mild up/down hill angles. It's surprisingly difficult to eyeball what's level and estimate vertical angles. A good topo map with accurate placement of bench and target points helps. I have a small Locke style surveyor's hand level to establish what's horizontal, and an inexpensive plastic sextant for reasonably finding elevation angle. At 600 yards small vertical angles become more significant.
-

 

[Eagle Six]

I had to rush out for a scheduled ride and did that pretty quick, I'll double check and report back.

 

[Eagle Six]

I really hate given misinformation based on me being wrong, but apparently I was wrong and edited my previous post to reflect that. I really hate being wrong and not being able to determine how I come to a wrong conclusion, but I have no idea at this point how I came up with 5.75 MOA up!

I have entered, run, and re-entered three times. I think I have it correct now and it circles back to what you (@17Hornet) stated in your very first post.

G7 .235 with a Zero Height of 0", 8.25 MOA Up
G7 .235 with a Zero Height of .325", 8.0 MOA Up
G7 .266 with a Zero Height of 0", 8.0 MOA Up
G7 .266 with a Zero height of .325", 7.75 MOA Up

NOTE 1: I did not check Hornady for the G7 BC, however AB say's it's .238 and that makes no difference in this example.
NOTE 2: When I use the custom BC from AB, it makes no difference for this short distance calculation.

I wonder if @SKWERLZ would be kind enough to recheck his data.

So, now that my AB agrees with JBM and your initial post, I'm wondering if the difference in the calculated trajectory and what your friend experienced may have been caused with an issue around the elevation turrets?

(George thinking to himself; how could you be so damn lame to post that 5.75 MOA, what an idiot! Now slap yourself a few good ones and apologize?!?!?)

 

[SKWERLZ]

I really hate given misinformation based on me being wrong, but apparently I was wrong and edited my previous post to reflect that. I really hate being wrong and not being able to determine how I come to a wrong conclusion, but I have no idea at this point how I came up with 5.25 MOA up!

I have entered, run, and re-entered three times. I think I have it correct now and it circles back to what you (@17Hornet) stated in your very first post.

G7 .235 with a Zero Height of 0", 8.25 MOA Up
G7 .235 with a Zero Height of .325", 8.0 MOA Up
G7 .266 with a Zero Height of 0", 8.0 MOA Up
G7 .266 with a Zero height of .325", 7.75 MOA Up

NOTE 1: I did not check Hornady for the G7 BC, however AB say's it's .238 and that makes no difference in this example.
NOTE 2: When I use the custom BC from AB, it makes no difference for this short distance calculation.

I wonder if @SKWERLZ would be kind enough to recheck his data.

So, now that my AB agrees with JBM and your initial post, I'm wondering if the difference in the calculated trajectory and what your friend experienced may have been caused with an issue around the elevation turrets?

(George thinking to himself; how could you be so damn lame to post that 5.25 MOA, what an idiot! Now slap yourself a few good ones and apologize?!?!?)

I mistook the OP distance for 500

 

[Eagle Six]

I mistook the OP distance for 500

That is the best I can come up with is looked at the 600 yard row on the chart, but read the 500 yard row!

Again my apology to @17Hornet for screwing that up!

 

[17Hornet]

That is the best I can come up with is looked at the 600 yard row on the chart, but read the 500 yard row!

Again my apology to @17Hornet for screwing that up!

All good guys! Thank you both for taking the time to check and recheck your data for me. When you stare at a chart long enough it all just kinda blurs together.

Right now the best I can come up with is an updraft, combined with the zero height and perhaps a small tracking error in the optic.

I will report back with an update next weekend (hopefully).