Dope question

[omicron1792]

Shooting 6.5prc. 20.5” barrel. AB raptor 8 stack.

140gr tipped federal fusion. Box says 2975. I got 2835 with magneto speed v3 on 40 shots. SD 12. Listed BC of .529

My dope using 2 different well known apps was dead on at 200 and 300 yards after a 100 yards zero. I shot a 10 shot group at 100 with around 1.25 MOA.

At 600 yards I was off by one MOA. I confirmed this with multiple shots.

Should I change ballistic coefficient in apps in order to make this match? Or should I change velocity even though I measured it with a chronograph? I have to change the ballistic coefficient up to about 6.5 to get it to match what I shot at the range. I know it really doesn’t matter which one of these two I change, I am just curious what most people do

 

[ShiddyPiddy]

I wouldn't change anything based on one session. There are lots of factors that could be contributing to it being 1 MOA off. Head/tail winds, target at a slope, etc. I use the apps to get me ball park and write down my dope for future reference and field use.

 

[skipper907]

how are you off ? left or right ? Up or Down ?
Left right I would adjust the scope
up down I would first check your inputs into your app

 

[omicron1792]

how are you off ? left or right ? Up or Down ?
Left right I would adjust the scope
up down I would first check your inputs into your app

Up down. My windage stayed correct throughout the session.

Apps said 12.6 moa. Actual on target was 11.25.

I confirmed at another range they had there with varying distance animals. My solutions were all correct from app on sub 500 shots.

For an elk target at 617 yards 11.75 was correct. App solution was 13.

 

[Macintosh]

What app?
I would make certain all inputs are correct (including angle to target, DA, etc), and then after verifying dope at range with a largish group (10+), velocity true the app according to the instructions. Not sure how your app does it, my AB gives space for 3 truing distances which plots the velocity and the changing bc on a curve—this is because the bc changes as a function of velocity, the bc isnt a constant, and the published BC is usually valid for close range (0-300ish). A different bc at slower speeds will affect the velocity at middle distances even if your mv is correct. So even if it gives you a “false” muzzle velocity based on the actual dope, the trued velocity will be closer to your actual dope averaged across that bullet path.

Curious what folks with more experience say, what I outlined above is what has worked for me, BUT…my understanding of it may be off.

 

[skipper907]

was your group above your point of aim or below, when zeroing and at distance ?
your using the average velocity you measured ?
changing your BC from .529 to .65 as you said to get it to line up would be an extreme amount of adjustment. Indicates app value being off. however the bc could be off some considering those aren't a match bullet

 

[omicron1792]

I shot lots of groups. The range officer was with me whole time because I was only one there.

It’s weird because again at 2 and 300 I was dead on.

Using ballistics calculator and SBC lite.

This is a world class marksmanship range. There were really very little angles to account for

 

[omicron1792]

Only thing I can figure is this is newer projectile. Federal put a tip on their fusion line. According to their numbers it only increased their BC a little bit. Maybe they were very conservative in their BC measurements.

The app inputs are correct.

6.5mm
140gr
2835fps
0.529bc
2.1” sight height

@Formidilosus What are your thoughts

 

[Formidilosus]

Only thing I can figure is this is newer projectile. Federal put a tip on their fusion line. According to their numbers it only increased their BC a little bit. Maybe they were very conservative in their BC measurements.

The app inputs are correct.

6.5mm
140gr
2835fps
0.529bc
2.1” sight height

@Formidilosus What are your thoughts

The BC isn’t moving up to nearly .7 G1. I would be very leery of that. Seems something else is off.

 

[pattimusprime22]

Another thing to check would be scope height over bore.

 

[SloppyJ]

Something seems off for sure. When I adjust velocity I'm typically under a 25fps delta. That's not going to do it for you.

However, I've shot with people who have adjusted their velocity over 100fps. If I go more than 25fps I start triple checking things.

I've changed BC up to 20 thousandths or so as well. What you're describing would have to be more than that.

 

[skipper907]

I would try looking at your temp input or inputs and the elevation and station pressure inputs and set your dew point to 50%