Weird Error In Ballistic Calculation

[BallisticsGuy]

Could be a dead click or 2 in your scope. Would not be the first time I've seen that before.

Have you tried doing it without dialing, just using hold-off? Would need to do that to eliminate the possibility.

 

[PBR driver]

I had the opportunity to test out my 7mm WBY magnum at extended range last weekend. I use JBM ballistics to calculate my dope... However while shooting I experienced a strange error in the calculation of my drop. I'm going to provide as much information as possible and I hope somebody can chime in, because I am now clueless.

DATA

Nosler 160gr. Accubond @3240 (10yds from chronograph)
Scope Elevation 1.561"
35 degrees , cloudy, wind (5-10 <----)
Elevation 1600' Pressure 29.7
Zero'd @ 275 yards
Shooting mountain to mountain (over a dry dam)

The ranges I shot were 435 yards, 575 yards, 700 yards, and 835 yards. The elevation data was very close for all of the ranges except for the 575 yard elevation data. I used other various free calculators and got the same results... I even played with the input of the ballistic data, muzzle velocity, zero, with various combos and the 575 yard data was always off...I'm stumped!

Range (SUGGESTED DROP) (ACTUAL DROP)
435 yards - 3 MOA - 3.25 MOA
575 yards - 6.3 MOA - 5.25 MOA
700 yards - 9.6 MOA - 9.25 MOA
835 yards - 13.5 MOA - 13.5 MOA

Thanks in advance for your input.

Remember, a ballistic calculator is just calculations, or in other words theoretical, when you shoot you are taking theory and preforming the practical .
Your mileage may vary from a calculation.
I have found that my calculator maybe spot on but sometimes our scopes may not be as precise as needed.
At 100 - 500 yards you maybe very close but scope adjustments may not be if you have a custom turret to be dialed in tight from 500 on out.

 

[WiscGunner]

Thank you for providing so much pertinent info that is so often lacking is these type of questions. You seem to have a number of factors that are likely at play but I would like to put your data into my app to try to find which error is affecting you the most as I too am curious.

Likely the up/down draft, as many have already mentioned, is compounding your issues but the long zero range is not helping you at all and your scope height seems awfully low. Perhaps you have measured it correctly but I all to often see people input this inaccurately (ring height, tube over barrel, etc) and not center of bore to center of scope tube.

I have a couple more questions though in an effort to replicate your data.

Is this a factory rifle or custom?
What is your barrel twist rate?
What BC are you using for your bullet?
What model scope are you using?
What is the SD of your ammo?
Were you using a true range or an adjusted range (angle compensated)?
Have you done a full tracking test on this scope before?

 

[BLC3128]

Thank you for providing so much pertinent info that is so often lacking is these type of questions. You seem to have a number of factors that are likely at play but I would like to put your data into my app to try to find which error is affecting you the most as I too am curious.

Likely the up/down draft, as many have already mentioned, is compounding your issues but the long zero range is not helping you at all and your scope height seems awfully low. Perhaps you have measured it correctly but I all to often see people input this inaccurately (ring height, tube over barrel, etc) and not center of bore to center of scope tube.

I have a couple more questions though in an effort to replicate your data.

Is this a factory rifle or custom?
What is your barrel twist rate?
What BC are you using for your bullet?
What model scope are you using?
What is the SD of your ammo?
Were you using a true range or an adjusted range (angle compensated)?
Have you done a full tracking test on this scope before?

WiscGunner,

Thank you for your response, here is the data you requested:

• It's a Weatherby Mark V (Japanese Made) with a round count of ~350. The action is bedded including the first 2" of the barrel with pillars installed as well.
• The barrel is 26" long with a 1-10" twist.
• I normally use the G7 number for my BC, however I could not find reliable information for a .284 160gr Accubond. So I used the G1 BC
• My scope is a Leupold VX-5HD mounted to leupold 2 piece base with leupold rings.
• The SD of my reloads is 4.5.
• I am using an angle compensated range (Vortex Rangefinder has it built in).
• I have not performed a full tracking test.
• When I calculate my height of scope I calculate my height of the center of the scope to the center of the bore.. (1/2 bolt diameter + 1/2 scope tube diameter + measurement from the top of the bolt to the bottom of the scope.)

On a side note - I'm open to all suggestion, however I cannot grip the fact that scope tracking is the issue. The adjustment for the closest target and farthest target was very accurate. I thought perhaps stability with the 10" twist was an issue, however I'm pushing the 160gr bullets north of 3240 (JBM ballistics suggests my true MV is 3259 because I recorded 3240 10yd from the muzzle).

For a factory gun with a 1.5 MOA guarantee, I am satisfied with the accuracy so far. Here are the results of last weeks range trip.

I am shooting of a solid bench with a benchrest style front rest and heavy rear sandbag.

435 yards, 4 shot .75MOA group @ 435 and if you add in the 3 I considered poor-technique-flyers, the group opens to 1MOA.

575 yards, once I recalculated my elevation, I shot a 3 shot 1 MOA group.

700 yards, I only took 2 shots due to low ammo count! 2 shots within 3" of eachother.

835 Yards, I shot a 10" 4 Shot group with a stiff wind.

 

[WiscGunner]

WiscGunner,

Thank you for your response, here is the data you requested:

• It's a Weatherby Mark V (Japanese Made) with a round count of ~350. The action is bedded including the first 2" of the barrel with pillars installed as well.
• The barrel is 26" long with a 1-10" twist.
• I normally use the G7 number for my BC, however I could not find reliable information for a .284 160gr Accubond. So I used the G1 BC
• My scope is a Leupold VX-5HD mounted to leupold 2 piece base with leupold rings.
• The SD of my reloads is 4.5.
• I am using an angle compensated range (Vortex Rangefinder has it built in).
• I have not performed a full tracking test.
• When I calculate my height of scope I calculate my height of the center of the scope to the center of the bore.. (1/2 bolt diameter + 1/2 scope tube diameter + measurement from the top of the bolt to the bottom of the scope.)

On a side note - I'm open to all suggestion, however I cannot grip the fact that scope tracking is the issue. The adjustment for the closest target and farthest target was very accurate. I thought perhaps stability with the 10" twist was an issue, however I'm pushing the 160gr bullets north of 3240 (JBM ballistics suggests my true MV is 3259 because I recorded 3240 10yd from the muzzle).

For a factory gun with a 1.5 MOA guarantee, I am satisfied with the accuracy so far. Here are the results of last weeks range trip.

I am shooting of a solid bench with a benchrest style front rest and heavy rear sandbag.

435 yards, 4 shot .75MOA group @ 435 and if you add in the 3 I considered poor-technique-flyers, the group opens to 1MOA.

575 yards, once I recalculated my elevation, I shot a 3 shot 1 MOA group.

700 yards, I only took 2 shots due to low ammo count! 2 shots within 3" of eachother.

835 Yards, I shot a 10" 4 Shot group with a stiff wind.

Thank you for the extra information. It sounds like you have pretty accurately set up your system.

The one thing I would strongly suggest you do is forget you ever heard of Angle Compensated Range, this has no use outside a classroom. This will add hidden error to your system for no reason. Angle Compensated Range only affects elevation corrections in a vacuum. The big downside of this is that the bullet is still traveling the entire distance through environmentals even if on paper says it has a shorter path. It is basically a lie that deceives the truth of bullet flight.

Did you get these environmentals onsite (Kestrel or WeatherFlow) or from a nearby weather station? It is best to get them on site as actual data as there can be significant differences from the nearest weather tower.

What was the humidity? When figuring Density Altitude (how thick the air is) for ballistics it is important to have all 3 (Air Pressure, Temperature, Relative Humidity). I really like BallisticARC (free/14.99PRO) with a WeatherFlow weather meter ($80). Simple system with reliable data and easy inputs in the field including shot angle wind/shot bearing.

A 100yrd zero will give you an actual zero with almost no added variable of weather. Then, with your accurate velocity for that days temperature, you can start to dial in the BC of the factory bullet (on a flat range with little to no wind) as it actually performs in your rifle. This is affected by , bore (size, finish), muzzle turbulence (brake, crown, suppressor), chamber concentrically and velocity and will continue to change as velocity changes hence the increasing use of banded BC's.

Then you will have proper data to start determining what the terrain environmentals are adding to you equation.

What direction was the 5-10mph wind blowing? This can affect elevation even on a flat range depending upon the direction related to the shooter (12:00-ish, 6:00-ish).

Adding in steep terrain and a canyon you will have compounding issues. Wind is generally faster higher off the ground so crossing a depression can many times have fast wind than is registering on vegetation or at the shooter height. Then you have the wind direction against the terrain that can result in up-drafts/down-drafts. Then lastly you have temperature changes from cool air in the morning warming and rising toward afternoon to warm afternoon air cooling and sinking in the evening. The heating and cooling air creates thermal updrafts and down draft over the terrain.

Hope something in here was mildly helpful.