DOPE issues compared to ballistics calculators

[DT1993]

I am shooting hornady factory 6.5 Creedmoor 147gr ammo. Up to 600 yards or so, results from both the Hornady 4DOF app and Applied Ballistics line up with my actual POI. However, at 685 yards both apps call for right around 4.4MRAD and my actual groups are 0.3MRAD below that. At 800 yards, both apps spit out around 5.8MRAD and my actual groups are 0.6-0.8MRAD low. All shooting has been done prone with a bag at distance, and the groups are around 0.5 MOA. My muzzle velocity (with 20 rounds down the barrel) is 2736. I am now at 100 rounds on the barrel. Any idea what might be going on? I have confirmed that all atmospheric data, muzzle velocity, sight height, twist rate is correctly input. Any help is appreciated!

 

[OREGUN]

Time to true your BC.

basically, adjust the MV in your computer to make it match your actual POI inside of 600 yds. Beyond 600 adjust BC input to make your computer match actuals.

there are probably 1000 pages on the topic if you search the site. Frank’s ( @lowlight )book does a good job of describing it too.

 

[longgunJR]

have you trued your velocity and BC? if of so have you tracked your scope?

 

[DT1993]

Time to true your BC.

basically, adjust the MV in your computer to make it match your actual POI inside of 600 yds. Beyond 600 adjust BC input to make your computer match actuals.

there are probably 1000 pages on the topic if you search the site. Frank’s ( @lowlight )book does a good job of describing it too.

have you trued your velocity and BC? if of so have you tracked your scope?

I have attempted to true MV and had to change it over 150fps to get the 800 yard result I need, but it threw everything closer off significantly. I will have to try to balance the changes of both and see if I can make it work

 

[ma smith]

OP what is ∆ come ups for all of these ?
and eg 600 to 685 is come up of x
and variance of y

 

[DT1993]

OP what is ∆ come ups for all of these ?
and eg 600 to 585 is come up of x
and variance of y

810 yards come up is 5.77, variance was ~ 0.7 (in reality dialed 6.5 for dead center hit)
685 come up is 4.4, variance was 0.3 (dead center impact with 4.7)

 

[OREGUN]

I have attempted to true MV and had to change it over 150fps to get the 800 yard result I need, but it threw everything closer off significantly. I will have to try to balance the changes of both and see if I can make it work

You said you were good to 600 yds...so your MV is ok. Beyond 600, things fall apart. Meaning, it’s likely that your actual BC is not as good as whatever published value you put in the computer. Adjust that input until what is called on the computer matches what you actually needed. This is assuming your scope tracks truly. Probably not a bad idea to turn off spin drift, Coriolis Effect and aerodynamic jump as able on your computer. My computer tends to over-value those various effects.

 

[DT1993]

Adjusting my BC seems to make the predictions in the 400-700 yard range higher than reality (ex 450yards calls for 0.2 mils higher than needed). I guess I will have to do a tall target test and ensure my tracking is on. I’m using a Razor HD Gen II 4.5-27x56

 

[DT1993]

You said you were good to 600 yds...so your MV is ok. Beyond 600, things fall apart. Meaning, it’s likely that your actual BC is not as good as whatever published value you put in the computer. Adjust that input until what is called on the computer matches what you actually needed. This is assuming your scope tracks truly. Probably not a bad idea to turn off spin drift, Coriolis Effect and aerodynamic jump as able on your computer. My computer tends to over-value those various effects.

 

[ma smith]

810 yards come up is 5.77, variance was ~ 0.7 (in reality dialed 6.5 for dead center hit)
685 come up is 4.4, variance was 0.3 (dead center impact with 4.7)

It s sounds like you are off 0.3-0.4 every ~100 yds
eg between 600 to ~700 (685) you're off by .3 per 85yd
and again between ~700 (685) to ~800 (810) you are off by .4 per 125 yd

600 = X (actual X'=X -> 0.0∆)
685= 4,4 (actual 4,7 -> 0.3∆)
810= 5,8 (actual 6,5-> 0.7∆)

What was your good dope for 600yd?

 

[DT1993]

It s sounds like you are off 0.3-0.4 every ~100 yds
eg between 600 to ~700 (685) you're off by .3 per 85yd
and again between ~700 (685) to ~800 (810) you are off by .4 per 125 yd

600 = X (actual X'=X -> 0.0∆)
685= 4,4 (actual 4,7 -> 0.3∆)
810= 5,8 (actual 6,5-> 0.7∆)

What was your good dope for 600yd?

3.6 mils and my impacts were dead on

 

[OREGUN]

Definitely check the scope for accuracy. Looking at the tracking test data for those scopes, it’s pretty unlikely there is a problem there but it’s worth checking. Recently, I checked two scopes, one a tier one, >$3000 scope and one a tier 2.5 (made by a reputable manufacturer but at the low end of their line) ~$800 scope. The tier one is dead on all the way to the end of the adjustment range. The cheaper one was on for the first third of its range and then progressively overshot as it went up from there. back to your issue. Is there a wind component in your data that could be adding tenths of up for aerodynamic jump? The fact that your dope is matched with the computer up to 600 and then, farther out, your computer is not calling for enough adjustment, leads me to think that your trajectory is decaying faster than the computer thinks it will. What form factor are you using, G1 or G7? How did you adjust it? Did you leave the muzzle velocity value alone (with whatever worked up to 600) when you changed the bc? What does Hornady use in place of form factor? I don’t know dick about the 4DOF model.

Some one on here knows way more about this than me. But I’m inquisitive and bored.

 

[DT1993]

Definitely check the scope for accuracy. Looking at the tracking test data for those scopes, it’s pretty unlikely there is a problem there but it’s worth checking. Recently, I checked two scopes, one a tier one, >$3000 scope and one a tier 2.5 (made by a reputable manufacturer but at the low end of their line) ~$800 scope. The tier one is dead on all the way to the end of the adjustment range. The cheaper one was on for the first third of its range and then progressively overshot as it went up from there. back to your issue. Is there a wind component in your data that could be adding tenths of up for aerodynamic jump? The fact that your dope is matched with the computer up to 600 and then, farther out, your computer is not calling for enough adjustment, leads me to think that your trajectory is decaying faster than the computer thinks it will. What form factor are you using, G1 or G7? How did you adjust it? Did you leave the muzzle velocity value alone (with whatever worked up to 600) when you changed the bc? What does Hornady use in place of form factor? I don’t know dick about the 4DOF model.

Some one on here knows way more about this than me. But I’m inquisitive and bored.

The wind was like 5 mph at a 90 degree angle. Hornady does use a form factor, but it isn’t adjustable enough to have correct impacts at 800 yards. I have changed muzzle velocity alone, and also adjusted it and the g7 BC trying to find an accurate solution. The Hornady model and applied ballistics are very similar with drops out to 1000 yards from what I can see

 

[ma smith]

Agree actual 800 dope is dropping like rock..is there some other explanation?
Maybe stability? or a ranging error? metric/english conversion or something...?

 

[OREGUN]

Are you using the g7 number that Hornady publishes for their 1.75 Mach BC? For the 147 ELD -Match it looks like they are saying .321.

 

[DT1993]

Are you using the g7 number that Hornady publishes for their 1.75 Mach BC? For the 147 ELD -Match it looks like they are saying .321.

I have tried using that number, as well as using the Doppler data that Hornady claims for their bullets using their app

 

[OREGUN]

Just out of curiosity, have you tried using the G1 value (.637) in the AB software? I think AB tends to favor the G7 model but it’s worth a try. Like I said, no idea about the 4DOF form factor numbers.

back to the .321 g7, what do you have to adjust it to to get the 800 computer calc to match actual assuming you left the MV value alone?

 

[DT1993]

Just out of curiosity, have you tried using the G1 value (.637) in the AB software? I think AB tends to favor the G7 model but it’s worth a try. Like I said, no idea about the 4DOF form factor numbers.

back to the .321 g7, what do you have to adjust it to to get the 800 computer calc to match actual assuming you left the MV value alone?

i have not tried the g1 but I will. It took dropping the MV to 2600 or the BC to 0.275ish , but it threw the data for 500-700 off by 0.4 mils

 

[DT1993]

Just out of curiosity, have you tried using the G1 value (.637) in the AB software? I think AB tends to favor the G7 model but it’s worth a try. Like I said, no idea about the 4DOF form factor numbers.

back to the .321 g7, what do you have to adjust it to to get the 800 computer calc to match actual assuming you left the MV value alone?

The g1 is actually closer to the 800 than the g7 was. Still 0.3 mils off but closer. A combination of using the g1 and lower the mv by a little may be the ticket

 

[OREGUN]

I just did some number crunching with my kestrel. I had to lower my .307 bc down to .223 to get the computer to spit out an additional .7 mils at 800 yards. You can see that my 4/5/600 values were off by .13, .22, and .34 after I did that.
I think, and this is kind of an assumption on my part, that through a combination of adjusting the bc and the muzzle velocity in tandem, you should be able to get your data aligned within a reasonable margin of error....I’ve never seen a hard answer about what “reasonable” is for this. I guess maybe a tenth...which is well inside an MOA. If G1 aligns better, use that.
One other thing, is your actual DOPE precise? We’re you just recording any hit on a 36” plate at 800 or were you aiming at a 2” truing bar painted on there?

In my limited experience doing this stuff, it doesn’t sound like there’s anything broken. Just remember that the bullet doesn’t lie. If you have accurate, precise DOPE at as many distances as you can get, make sure your inputs into the computer regarding caliber, weight, twist, sight height, etc are as precise as possible and then do whatever is required to get the computer to match your gun.

Of course, it’s entirely possible that someone smarter than me has a better answer for you.

 

[DT1993]

I just did some number crunching with my kestrel. I had to lower my .307 bc down to .223 to get the computer to spit out an additional .7 mils at 800 yards. You can see that my 4/5/600 values were off by .13, .22, and .34 after I did that.
I think, and this is kind of an assumption on my part, that through a combination of adjusting the bc and the muzzle velocity in tandem, you should be able to get your data aligned within a reasonable margin of error....I’ve never seen a hard answer about what “reasonable” is for this. I guess maybe a tenth...which is well inside an MOA. If G1 aligns better, use that.
One other thing, is your actual DOPE precise? We’re you just recording any hit on a 36” plate at 800 or were you aiming at a 2” truing bar painted on there?

In my limited experience doing this stuff, it doesn’t sound like there’s anything broken. Just remember that the bullet doesn’t lie. If you have accurate, precise DOPE at as many distances as you can get, make sure your inputs into the computer regarding caliber, weight, twist, sight height, etc are as precise as possible and then do whatever is required to get the computer to match your gun.

Of course, it’s entirely possible that someone smarter than me has a better answer for you.

I was aiming at a waterline on the plate

 

[ptosis]

Check if G7 BC = 0.333 fits your data any better. That's the value I've got from recent measurements using Labradar (and I have a feeling that "official" BCs and Cd-based tables that Hornady publishes for this specific bullet are... to put it politely -- misleading.)

 

[DT1993]

I had to go down to a 0.268 G7 to get things to line up, which seems excessive to me. There must be something else at play. I’m going to do a tall target test and re-chrono tomorrow if the weather cooperates

 

[ptosis]

I had to go down to a 0.268 G7 to get things to line up, which seems excessive to me. There must be something else at play. I’m going to do a tall target test and re-chrono tomorrow if the weather cooperates

Cool, please keep us posted.

(Also -- this may sound silly, but it did happen to me -- distance measurements may be worth double-checking too.)

 

[DT1993]

For any interested.....I shot again today. My muzzle velocity after re-chrono-ing was found to be 2686. I used the tall target test and found my "scope correction factor" for applied ballistics to be 0.993. True DOPE vs app was as follows. 800 true 6.4 app 5.9= 0.5mils off, 700 true 5.2 app 4.8= 0.4mils off, 600 true 4.2 app 3.7= 0.5mils off, 500 true 3 app 2.7= 0.3mils. All groups were around 0.5moa. I know the earlier suggestions were to true MV at 600 and BC at 800, but I am having a hard time making that work in applied ballistics with the 147gr ELDMs (which I have a stockpile of so changing ammo isn't in the cards). Any further suggestions or help would be greatly appreciated.

From a new long range shooter...Thanks!

 

[ptosis]

the angular error looks relatively constant with distance. this may be a [another] silly question, but did you check the (a) zeroing and (b) zeroing distance? considering the precision of your click value measurements, I would guess that you did.
other things to double-check would be atmosphere, especially temperature (but -- with rest correct -- it is unlikely to cause 0.5 mrad diff at 600 yd), and distances (maybe some systemic error -- wrong units?)
other than that, I am out of ideas (but am very interested to know the cause).

 

[DT1993]

the angular error looks relatively constant with distance. this may be a [another] silly question, but did you check the (a) zeroing and (b) zeroing distance? considering the precision of your click value measurements, I would guess that you did.
other things to double-check would be atmosphere, especially temperature (but -- with rest correct -- it is unlikely to cause 0.5 mrad diff at 600 yd), and distances (maybe some systemic error -- wrong units?)
other than that, I am out of ideas (but am very interested to know the cause).

I confirmed zero while doing the tall target test at 100 yards, confirmed with 2 rangefinders. One rangefinder only does 400 yards so the other distances were ranged and then “confirmed” using gps waypoints (I know not perfect but were within 5 yards of the rangefinder). I have a cheaper kestrel model 2500 that does not do humidity but everything else seems to be consistent with data from a weather station located just a couple miles away

 

[ptosis]

Thanks! (I admit I'm stuck; will just hope someone more experienced finds the root cause.)

 

[Mike Ryan]

Let me preface this by stating 1. I shoot 175 grain .308s, 2. I use GeoBallistics for software, and 3. I am still a rookie at this. I had a similar issue with my RPR past about 600 yds and I was able to simply put in the truing dope and save it (actual come up and range). It did not cause any issues at the shorter distances but did correct everything out to about 800 yds. I haven't gone further than that. I did learn that the software is for the bullet only and on GeoBallistics, it lists three BC's for your 147 gr ELD's for G7 drag function:

0.351 for MV 2512 - 5000
0.332 for MV 1953 - 2512
0.321 for MV 0 - 1953

I don't know if AB or the others do that as well, I assume they would. Perhaps if you changed the BC that corresponds to the MV at the given range and not the BC for the shorter ranges it may help. Based on Hornaday's data, you are likely into the mid level BC at 600-800 yds. Also, Hornaday's test data is for 24" barrel, but that one is pretty obvious. Just a shot in the dark....

 

[ma smith]

My guess is the data is 99% OK, but perhaps is stacking 1% errors in RF, Tall Target, BCs etc.
Here is some minor tweaks and run thru web version of Berger Ballistics...

[Estimated] 685= 4,4 (actual 4,7 -> 0.3∆)
[Estimated] 810= 5,8 (actual 6,5-> 0.7∆)

Note on the innacuracies of Hornady BCs...
(https://www.hornady.com/support/ballistic-coefficient)

Maybe somebody smarter can chime in on what is really going on,
this is just forcing the numbers to fit to have a discussion...

 

[DT1993]

I know I’ve been focusing the discussion on applied ballistics, but I just lowered my MV by 15 and changed the axial form factor to the max of 1.1 in the Hornady 4DOF app and am 0.1-0.15mils off from 100-800 yards. I’ll have to confirm at 1000 to see but I may have found a temporary solution until I can get applied ballistics dialed in