Ballistics matching to a BDC reticle

Discussion in 'Reloading Room' started by Branth, Oct 30, 2014.

  1. Branth

    Branth Member

    6,275
    4
    So, I got a new scope recently (Primary Arms 1-6X) and it comes with this really awesome BDC reticle that is supposedly good all the way out to 800 yards. The problem is, it's only good for certain kinds of ammo, so I decided I needed to start working on making my own loads to match it. I think it's a neat system I've worked up, so I figured I'd share it, and if anyone has suggestions to how I can improve it, please let me know and I'll give it a try!

    I tracked down somewhere online where someone got data on how many MOA each of the BDC levels were calculated for, and got to work with some ballistics programs and spreadsheets. I put in the MOA drop on the BDC in one column on the spreadsheet, and then I opened up Sierra's website to pull BC info for the bullets I want, and then Hornady's ballistics calculator to get bullet drop data, and started fiddling around with it to get bullet drop data. I put that in another column next to the BDC data, and put them on a graph so I can get a visual representation of the bullet path. I then fiddled with muzzle velocity and zero distance to get the bullet path to match the BDC values as closely as possible. I did this with several bullets, and once I got them as close as I felt was possible, I started looking online to see if it would be possible to get that kind of performance. That's a whole other ball of wax that involved searching online for data on chrono data with my barrel length, comparing it to published data and muzzle velocities, and trying to ballpark how much increases in powder would increase muzzle velocities, but it's all back-of-the-napkin stuff until I finally buy a chronograph and get real-world data.

    Anyway, here's some of the work I did. Here are the bullet drop tables and one of the final loads plotted against the bullet drop. You can see that on the right side of the tables I calculated the difference between the reticle subtensions and the projected bullet drop, both in MOA and in actual inches on target.
     

    Attached Files:

  2. ajole

    ajole Supporting Member

    34,524
    10,762
    NE Utah
    Really nice work...and you use the word "subtensions! BONUS POINTS!!!

    The only criticism I have is the label of the graph..."Flight Path" makes it look like it goes higher as it reaches out, but its actually the "Holdover in MOA", right?
     

  3. Branth

    Branth Member

    6,275
    4
    ^truth. I'll edit it in my document.

    Another fun word: Steradian. It's like a radian(angle, metric version of MOA) in 3D - I found it when trying to explain candela vs lumens when talking about flashlights. Apparently, one candela is one lumen per steradian! Who knew?

    When you're talking about accuracy, I believe you could make the argument that we should describe accuracy in steradians rather than MOA, since MOA is only two-dimensional. MOA is suitable for holdover and windage(one dimension), but not for talking about accuracy (both dimensions). This assumes accuracy is evenly distributed across both axes - Groups are circular shaped instead of having pronounced vertical or horizontal stringing.

    This would be horribly complicated and headache-inducing, and therefore not something I'd actually want to do, but would technically be more correct than what we are doing now.
     
    Last edited: Oct 30, 2014
  4. undeRGRound

    undeRGRound ROLL wif Da MOLE! Supporting Member

    25,156
    1,408
    INDY
    Wouldn't a steradian be like a curved nappe. describing a shape like a long narrow curved horn?
     
  5. Branth

    Branth Member

    6,275
    4
    Not quite sure what a nappe is, I'm afraid...

    A steradian would be like a long conical shape - What you usually think of when you think of picturing a gun's accuracy. This would be measured in very small fractions of a steradian.

    http://en.wikipedia.org/wiki/Steradian
     
  6. undeRGRound

    undeRGRound ROLL wif Da MOLE! Supporting Member

    25,156
    1,408
    INDY
    A nappe is half a cone.

    Think "ice cream cone" and that is actually a NAPPE!
    What we call a cone is actually a half-cone. Geometrically speaking.

    So I guess we are on the same thing!
     

    Attached Files:

  7. undeRGRound

    undeRGRound ROLL wif Da MOLE! Supporting Member

    25,156
    1,408
    INDY
    Keep going, I'm learning more here from you, than vice-versa :D
     
  8. Branth

    Branth Member

    6,275
    4
    Cool stuff - I learned something new! :p

    Anyway, going back to my original post, here was my method:

    Pick a fairly warm MV for your round, a 100 yard zero, and pull the BC of your bullet from a manufacturer's website. With Sierra, I ended up using their slower BC most of the time (they gave 3 values for different velocities, and since I was pretty close to the border of the highest velocity at the muzzle, I used the next lowest value). I plugged those values into the spreadsheet, and plotted a graph of the reticle values and the test load.

    If you are shooting consistently low, you need to move your zero distance out, and if you are shooting high, you need to move it in. If the curve is too steep on your test load, you need to increase your MV, and if you're too flat, you need to lower it.

    Some info I read online seemed to indicate that the relationship between powder charge and muzzle velocity is roughly linear (that is to say, if you increase powder by 10%, you increase muzzle velocity by 10%). I haven't been able to test it myself, so I have to take that with a grain of salt, but it sounds plausible. In order to estimate muzzle velocity, I found some data online that had chrono data for someone's handloads out of a 16" barrel. I then went to the Hodgdon website and looked up the muzzle velocity for their max and starting loads. To estimate how much extra MV I get per grain of powder, I used the following formula:

    (max MV-start MV)/(max charge-min charge)

    For IMR4064, it seemed to be between 50 and 60 fps/gr depending on the bullet weight. Let's call that our constant C. I would then take the charge and the MV that I got from the 16" chrono data and use the following formula:

    16"MV +((max charge-16"charge)*C)

    This has a number of potential issues, including differences in velocity from barrel to barrel, changes in the efficiency of powder burn in shorter barrels, and it assumes my C value formula works, which it very well may not. It's just being used as a rough estimate since I don't have any better way to get MV figures.

    My issue is that in order to match the reticle with a good-performing bullet like a Gameking instead of a match bullet, this method indicates I would need to have a pretty stout muzzle velocity out of my little 16" barrel. It looks like I could comfortably shoot 155gr Palma MatchKing bullets and match, but I can't get the 150gr GameKing to match it unless I hit 2750 FPS, which is uncomfortably warm out of a 16" according to what info I've been able to find online.
     
  9. ajole

    ajole Supporting Member

    34,524
    10,762
    NE Utah
    Just to clarify...why can you not get the 5 grain lighter bullet to go as fast at the muzzle, as the heavier bullet?

    Or is that 2750 a LOT faster than the Palma bullet's MV, in order to overcome the BC slowing the bullet faster downrange?

    Regardless, I can't imagine a 2700-2800 fps load would be an issue for a .308, even out of a 16" barrel. But it may take some tweaking with powders to keep it out of the max load area, which may be a BIG issue, these days.:(
     
  10. FlashBang

    FlashBang I Stand With Talon Lifetime Supporter

    Don't forget to account for atmospheric pressure, shot angle, temperature, powder used, cross wind and humidity. ;)

    Since I'm lazy, I just use QuickTarget that came with QuickLoad. I do the load data in Quickload, open Quicktarget and tell it to use my Quickload data, set the temp, etc. and it gives me the MOA adjustment needed and drop etc. for the load. :D

    .
     
  11. FlashBang

    FlashBang I Stand With Talon Lifetime Supporter

    Generating advanced load data involves more then increasing/decreasing powder charges. Case trim length, seating depth, type of primer, type of bullet, COL, barrel length and jump all add in to creating a load and determining its flight characteristics. I've really been seeing how everything all interacts since getting the Quickload software and running simulations as well as creating load data. I'm finding out just how little I really knew about advanced load development.... :p
     
  12. Branth

    Branth Member

    6,275
    4
    The 155gr Palma are super flat shooting, and match the BDC pretty closely at only 2575fps with a 220yd zero, whereas the 150gr GK drop faster, and need to be going 2750FPS with a 200yd zero.

    How much and in what direction do all of these influence muzzle velocity?

    I'm really not going as much for hairsplitting accuracy on this load as I have previously - My emphasis here is going to be in getting it to match the BDC reticle so I can hit reasonably big targets at range.
     
  13. ajole

    ajole Supporting Member

    34,524
    10,762
    NE Utah
    Yep, that's what I thunk.:p