Correcting zero for different loads out of the same rifle

[Mike Ryan]

This may be a foolish question, but in my twisted mind it makes sense. With ammo shortages and the like, I have a variety of 308 rounds that I would like to be able to use with my RPR without having to re-zero every time I change. The rifle is currently zeroed at 100 yds with 175 grain Federal GMM and I'd like to keep it there. I use Ballistics Arc's GeoBallistics software to get my elevation adjustments and have trued the software at 700 yds with the Federal GMM. One nice feature of the software is that it allows me to enter several different rifle configurations with different loads. It would be great to be able to just chose a different configuration from the software menu and have the elevation adjustments be accurate without having to mess with the zero on the scope. I ran my Federal GMM loads through a trajectory calculator (http://www.shooterscalculator.com/ballistic-trajectory-chart.php) with the zero distance set to zero and it tells me that the bullet drops 2.70 inches over 100 yds. I did the same thing for two other loads, a 147 grain and a 168 grain. I imported this data to a spreadsheet, graphed it, and fit a curve to each. I used the equation for that curve to determine at what distance I get exactly a 2.70 inch drop with each load. Turns out it is 106 and 103 yds respectively. Here's the question... If I put those values into the ballistics calculator as the zero distance for the rifle with that load, will that yield accurate elevation adjustments for the lighter loads? Any thoughts would be appreciated before I go waste a ton of ammo this weekend trying to prove my theory.

 

[demolitionman]

In AB, Create a separate bullet profile within your particular rifle list. you can simply enter your fps, zero height and your zero offset in the edit bullet menu, create it as its own profile. Then your are gtg and won’t have to re zero any different loads just adjust in your ballastic app zero offset and height.

 

[Steel head]

In AB, Create a separate bullet profile within your particular rifle list. you can simply enter your fps, zero height and your zero offset in the edit bullet menu, create it as its own profile. Then your are gtg and won’t have to re zero any different loads just adjust in your ballastic app zero offset and height.

many apps as well can easily accommodate this.

 

[Mike Ryan]

Thanks for pointing that out, I should have figured there was an easier way. In the GeoBallistics software, when I select a rifle, there's a section to enter the sight height, zero range, elevation offset, windage offset, twist rate and muzzle velocity. Would the elevation offset be the thing to adjust? The delta from the 175 grain to the 147 grain is 2.70 to 2.38 or -0.32 inches. Change the offset, the velocity and the bullet info and that should be it?

 

[addertooth]

On one of my rifles have a wheel (snail) on my parallax knob on my scope. I have ran narrow white pen striping tape on the outside of the wheel, with the holdover adjustment for different rounds, for when the parallax is correct. There is one stripe for each round with values written on that stripe. I only have to pick the strip which is for that round, set parallax, and read the value. I am too lazy to memorize this stuff, or look it up on a dope sheet. The wheel IS the dope sheet. This is especially useful on this rifle, as it is for use with subsonic speeds, and drop is more significant. But, this would also apply to higher velocities at longer ranges.

 

[demolitionman]

Thanks for pointing that out, I should have figured there was an easier way. In the GeoBallistics software, when I select a rifle, there's a section to enter the sight height, zero range, elevation offset, windage offset, twist rate and muzzle velocity. Would the elevation offset be the thing to adjust? The delta from the 175 grain to the 147 grain is 2.70 to 2.38 or -0.32 inches. Change the offset, the velocity and the bullet info and that should be it?

Yep. That’s it. Works in AB, Arc, 4dof or Shooter apps.

 

[theLBC]

Thanks for pointing that out, I should have figured there was an easier way. In the GeoBallistics software, when I select a rifle, there's a section to enter the sight height, zero range, elevation offset, windage offset, twist rate and muzzle velocity. Would the elevation offset be the thing to adjust? The delta from the 175 grain to the 147 grain is 2.70 to 2.38 or -0.32 inches. Change the offset, the velocity and the bullet info and that should be it?

i only have one rifle, but i have 4 profiles for different ammo, from 175gr fgmm to 147gr m80.
only velocity and bullet data should be different (weight, length and BC).
you can enter your different zero distances if you already worked that out and then see how well it calculates for farther out.
you can then make adjustments if needed to fit the real results.

 

[308pirate]

A data book works too

 

[Precision Underground]

Yep, bullet profiles. This is what I use for suppressed/un.