What am I missing??

[Halleywood]

I have no experience with ballistics programs, but I have been playing around with the G7 ballistics program and my STW info. I shot at 100 yards, then at 300 yards to get my drop and back into my velocity. My Dad has the chronograph so I can't read these loads. Well when I put in my zero at 100 and a 9 inch drop at 300 I'm getting 3043 velocity. Great! Well my 100 yard group was 1.5" high at 100 and my 300 yard group was 9" below that. So I was playing with the zero adjustment 1.5" high and -7.5" at 300. From that it gave me 2787 velocity!! My simple mind thinks that the bullet drop is 9" whether I have the zero at 100 or 1.5" high at 100. Why would it give me such a large discrepancy and what am I missing???? I wish I had the chronograph so I could just know my velocity.

 

[Lefty7mmstw]

The change in zero will magnify with range as you go out. a 1" difference at 100 is 3" at 300. You also need to take into account the scope to bore height; are you certain you are perfectly correct on that one?
How close are your ranges to the real distance? some ranges are a bit long or short and that will mess with your chi also.
I'd step a bit further out now for more accurate drops now that you know where you are at shorter range. I'd also wonder if your bullet's bc is correct.

 

[Halleywood]

Thanks for the reply John. I wouldn't say I'm 100% positive on the scope height. I'm close though. I actually thought about actual range after I was home from the range but no I didn't verify ranges. Unfortunately I don't have a place to shoot further until I get out to my parents place. Then I will have the chronograph and space to shoot as far as I am able. The bullets I'm running are 180 grain berger VLD. I haven't read anywhere that the bc values aren't accurate like on nosler.

I must be wrong in my thinking that my drop will be 9" no matter what my zero is at 100? Obviously this is my first attempt at ballistics programs!!

 

[Lefty7mmstw]

Ballistics Calculator - Hornady Manufacturing, Inc

using a 640 bc and 2900 fps I got a 6.6" drop (from zero) if zero'd at 200 and an 8.8" drop difference if zero'd at 100. I was guessing at the berger's bc but it is at least in the ballpark from what midway says. Keep in mind barrels, velocity, and altitude affect the bc you really get.

 

[Halleywood]

Berger has the BC at 659. I am expecting velocity to run around 2900-2950 in my rifle.

Once I am able to shoot over my chronograph (magnetospeed) and get my known velocity, then shoot at say 500-600 yards to get my true drop, do I adjust the actual BC to match the drop I actually shot?

So actual BC for the same bullet varies from rifle to rifle, location, etc? Good to know...I always thought that was the constant...thanks!!

 

[Lefty7mmstw]

Berger has the BC at 659. I am expecting velocity to run around 2900-2950 in my rifle.

Once I am able to shoot over my chronograph (magnetospeed) and get my known velocity, then shoot at say 500-600 yards to get my true drop, do I adjust the actual BC to match the drop I actually shot?

So actual BC for the same bullet varies from rifle to rifle, location, etc? Good to know...I always thought that was the constant...thanks!!

effectively, your bc goes up as the air thins with altitude, your barrel may distort the bullet more or less then the barrel used for the bc measurements, bc changes a bit with velocity (it won't even be the same all the way out in your rifle), and twist rate will change your bc a bit (especially slower than recommended twist) as the bullet can have a bit of yaw to it and hence a bit more frontal surface area.

bc really is relative...
At the ranges I usually shoot I make sure I have a pointy bullet with enough weight to carry well (produce decent bc) and leave the bc wrangling to those who consistently go really long. I'm more concerned with accuracy but don't want a blunt/stubby bullet screwing things up for me if I need to stretch a bit.

 

[Halleywood]

Good info...thanks