Help with ballistic calculator

[rclouse79]

Let me start by saying today was my first day shooting a scope with target turrets. My scope and ballistic app do not seem to be on the same page. I wanted to post what is going on in case I am doing something stupid. The scope is an Athlon midas tac and I am using the Hornady app. The adjustments on the scope seemed to be right on at 100 yards, which is what I zeroed it at.

I ranged the target at 405 yards and the Hornady ballistic app told me to go up 1.5 mil rad. I shot three times and was 6, 7, and 10 inches low for an average of 7.66 inches. I multiplied 405 yards by 36in/yd to get 14580 inches. I multiplied that by 0.0001 rad which gives me 1.45 inches per click at that range. Dividing the amount I was low by the amount each click should bring me up gave me 7.66/1.45 = 5.25 clicks. I started to play around with the velocity of the bullet so the app would give me 2.0 mil of adjustment at 405 yards. After seeing I had to change the velocity from 3000 to 2700 fps I started to think I had entered something wrong. I am shooting a 130 grain barnes ttsx with 54.5 grains of H4350 out of a .270 tikka. I have never actually measured their velocity with a chronograph, but can’t image they are that slow. I double checked everything I had entered: zero range-100, sight height-1.7 in, barrel twist-10, Ballistic Coefficient- 0.392, and G7 drag function. The environmental data matched where I was shooting. I am planning to go back out and shoot at different distances, but I wanted to get the app close before going back out. Any advice would be appreciated.

 

[lyle_destroys]

You need to chronograph your loads. Take that variable out of the equation. Using 4dof and accurate data I've made 1st shot hits out to 600 yards on 8" steel.

 

[pyrotechnic]

I don't think that the 2700 fps is all that unrealistic, however that BC sounds awefully high for a G7 bc. My 06 runs right around that speed and is right around 2 mils at 400yds.

At the shorter distances (lets say inside of 500) velocity is the driving variable for drop. So inside of that, tweek velocity to get the program to match your actual drop. If it's matching up well at the shorter ranges but diverging at distance then you can tweek the BC.

I would go back out. Shoot a tall target test to make sure your scope is tracking properly, and then shoot multiple distances as far out as you can. 400, 500, 600, 700 etc.

Write down the real world elevation required and the environmental conditions for which they hold true. Then with these known inputs( the big ones are :Sight height, Temperature, and Pressure) dick around with your app at your leisure to make it match. Make sure you know the difference between barometric and station pressure. That screws alot of people up starting out.

 

[pyrotechnic]

I just googled it. Barnes lists the .392 BC for the 130gr ttsx. It is a G1 BC and requires that you use the G1 drag function. The G7 BC will be closer to 0.19 or so. A huge BC change like that will change your numbers a bit even at the shorter distances. Just change your drag function to the G1 and I'm betting the numbers will start making a bit more sense. I would still do all the the above.

 

[rclouse79]

Thanks for all of the advice. I am planning to head out tomorrow and try some of it out.

 

[rclouse79]

You need to chronograph your loads. Take that variable out of the equation. Using 4dof and accurate data I've made 1st shot hits out to 600 yards on 8" steel.

I didn’t see the bullet I shoot in the 4dof program.

 

[Mtslackcountry]

Need to chrono that load, Also make sure BC Input is correct.

 

[rclouse79]

I just googled it. Barnes lists the .392 BC for the 130gr ttsx. It is a G1 BC and requires that you use the G1 drag function. The G7 BC will be closer to 0.19 or so. A huge BC change like that will change your numbers a bit even at the shorter distances. Just change your drag function to the G1 and I'm betting the numbers will start making a bit more sense. I would still do all the the above.

Thanks for the advice! I was hoping a post here would save me a bunch of banging my head against a wall. When I switched it to the G1 it changed the clicks to 1.8 which will be much closer. I thought I had picked the right option based on the shape of the bullet, but I guess not.

 

[rclouse79]

I don't think that the 2700 fps is all that unrealistic, however that BC sounds awefully high for a G7 bc. My 06 runs right around that speed and is right around 2 mils at 400yds.

At the shorter distances (lets say inside of 500) velocity is the driving variable for drop. So inside of that, tweek velocity to get the program to match your actual drop. If it's matching up well at the shorter ranges but diverging at distance then you can tweek the BC.

I would go back out. Shoot a tall target test to make sure your scope is tracking properly, and then shoot multiple distances as far out as you can. 400, 500, 600, 700 etc.

Write down the real world elevation required and the environmental conditions for which they hold true. Then with these known inputs( the big ones are :Sight height, Temperature, and Pressure) dick around with your app at your leisure to make it match. Make sure you know the difference between barometric and station pressure. That screws alot of people up starting out.

Do you think the pressure would cause much of a change at ranges inside 600 yards? I messed around with the app and entered huge swings in elevation, temp, etc., and it didn’t seem to change the number of clicks.

 

[pyrotechnic]

You're correct, at 400 yds you'll need very large atmosphere changes to to make a noticeable difference. Just like BC it's effects become much more pronounced the further out you get. The lower the projectile's BC the more pronounced these changes will be.

My above example with the 29.8" Hg with a 0 vs 4000 ft elevation input is a 0.3 mil change at 600 yds for my 06. That's not "huge", but it will be enough to make someone start chasing their tail if you're working backwards trying to get things to match up when guessing a muzzle velocity.

Ideally we want to only have one variable to change at a time. If we can measure our atmospherics we want to do so to eliminate as much error from that as possible. We already have an "unknown" mv we are trying to solve for and quite frankly I'm betting that the advertised BC is going to be slightly different from what you would end up with.

I just looked it up. My applied ballistics library lists the .277 140gr ttsx at a 0.191 G7 if you want to use the G7 function. The library lists .373 as the G1, since it differs from the advertised BC I would imagine they have measured it with doppler and that number is more accurate than the number barnes puts out. Since the ttsx isn't an incredibly sleek design I doubt you'll see much of an advantage going with the G7 profile.

 

[pyrotechnic]

Sorry I realized the "above example" in non-existent as I had it typed up as part of my first response then managed to delete it all because I'm an idiot and backed out of the browser.


The example was a difference between station (absolute) and barometric pressure.

Barometric pressure is corrected for altitude and requires both the pressure and altitude. Example: the weatherman says it's 29.8" Hg and you are at 4000ft elevation. You're station pressure would be closer to around 26.0" of Hg. This is what a pressure sensor or barometer would read (most smart phones have this sensor).

So your input into the app would either be

BOTH 29.8" Hg and 4000ft
Or
26.0" Hg with elevation set to 0 or "pressure is absolute" checked if that is an option.

ETA: the numbers aren't perfect in my example 29.8" Barometric @4000ft will probably not be exactly equivalent to 26.0" station pressure, but should be fairly close.

 

[rclouse79]

Sorry I realized the "above example" in non-existent as I had it typed up as part of my first response then managed to delete it all because I'm an idiot and backed out of the browser.


The example was a difference between station (absolute) and barometric pressure.

Barometric pressure is corrected for altitude and requires both the pressure and altitude. Example: the weatherman says it's 29.8" Hg and you are at 4000ft elevation. You're station pressure would be closer to around 26.0" of Hg. This is what a pressure sensor or barometer would read (most smart phones have this sensor).

So your input into the app would either be

BOTH 29.8" Hg and 4000ft
Or
26.0" Hg with elevation set to 0 or "pressure is absolute" checked if that is an option.

ETA: the numbers aren't perfect in my example 29.8" Barometric @4000ft will probably not be exactly equivalent to 26.0" station pressure, but should be fairly close.

Thanks for all of the charity. It is nice having a place to go for advice.

 

[pyrotechnic]

Let us know how it goes. If any other questions come up let me know. I nerd out on this stuff and am happy to help out.

 

[lyle_destroys]

I didn’t see the bullet I shoot in the 4dof program.

You will have to manually enter the data using the bc calculator in 4dof instead of picking your bullet from the list. That app took me a while to figure out all the way but it's now my favorite.

 

[rclouse79]

Think I am good. Shot at 200 with .4 mil and was right on. Shot at 410 yards with 2.0 mil with and was 3, 2.75, and 2 inches high. If I bump the velocity back up to 3000, which is what I thought it was based on my last bdc reticle, the app gives me 1.8 clicks. That would have lowered today’s group by 3 inches to put it right on. If I throw out my low shot yesterday 1.8 would have raised it by 4.5 inches, which would have been a couple inches low. Thanks again for the help. I really appreciate it.