A case for BDC Turrets

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by davkrat, May 18, 2011.

  1. davkrat

    davkrat Well-Known Member

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    I'm sure this dead horse has been beat plenty of times! I was running the numbers today to try and figure out which BDC settings to order my turret. The load is .308 Win, 150 gr ETips at 2850 fps. I decided to graph the click settings for a bunch of different altitude and temps.

    I've always thought it was funny when people complain about the slight differences you get when you change conditions. In reality out to 600 yards (many may call this medium range but I personally consider it my max) the differences for the various conditions are within a click up or down on a 1/4 MOA click scope. I even added an extreme condition of 10,000' at 60 degrees (not likely anywhere I hunt during deer season!) Don't know if this pic will show but looking at the data you'll see a BDC for 5,000' at 60 will theoretically work for 2,000' at 90 degrees and 8,000' at 30. You're within the one click zone for all those ranges and conditions.

    In other words a turret for 5,000' at 60 degrees needs 68.7" at 600 yards. That is just above the 68.6" for -1 click from the 2,000' @ 90 setting and below the 69.1" for +1 click from the 8,000' @ 30 setting. For all these conditions you should be within 1 click off which at 600 yards is less than 1.5" For big game hunting that seems like nothing to worry about. and having MOA comeups probably won't get you much closer due to the coarseness of the 1/4 MOA adjustment.

    [​IMG]
     
  2. Mikecr

    Mikecr Well-Known Member

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    I agree with you that for big game to 600yds, with that bullet & load, atmosphere is pretty minor stuff(compared to normal accuracy errors).
    If I were you, I'd pick a moderate condition expected and go with that solid.
     

  3. Fitch

    Fitch Well-Known Member

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    Nice work. I came to much the same conclusion when trying to decide what scope to put on my .30-06 and concluded that for deer, shooting the 150g bullet, a simple BDC reticle (Leupold VX-II with the LR duples reticle) worked really well out to 500 yards which is likely farther than I'll ever shoot at one here in PA. It's really simple and fast to use with ~70 year old eyes. If I were to go "out west", I'd want a range calibrated elevation knob on my 7RM.

    For varmint hunting (ground hogs in particular - which account for >90% of my annual hunting time) however, the situation is way different because of the difference in size of target circle. For Deer here I use 6", for ground hogs I use 1-1/2" for my Hornet or .17 Remington, 2" for my .223 (the terminal performance of the .223 is devastating compared to the other two - hit'em anyplace almost and they drop in their shadow). Trying to stay within 3/4" of LOS requires dealing with temperature and much more precise adjustment than trying to stay within 3" of LOS. So I pretty much hunt them within 300 yards or closer. PBR is 222 yards for the .223 but I can stretch it to 250 by holding the cross hairs on the top of the back when the hog is lying down feeding, or a generaous 300 by holding on top of the head with it sitting up.

    Fitch
     
  4. Len Backus

    Len Backus Administrator Staff Member

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    You are right. Especially at ranges under 600 yards, changing conditions don't affect ballistics enough to make it hard for BDC turrets to work well. And even if an adjustment is needed it is extremely easy at these ranges.

    And around 80% of shots are under 600 yards according to the 2,671 LRH hunters who answered this POLL.


    But this issue is like politics or religion to some so you'll never win the debate with some. :)
    .
     

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  5. Bart B

    Bart B Well-Known Member

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    Hope the bullet you use has good ballistic data for it. It needs 4, maybe 5 different values for the different velocity ranges it'll fly through the air at. Few bullet companies supply such data. And it needs to be based on actual firing tests producing the time of flight in different velocity ranges.

    And you'll need to chronograph your load to get a starting muzzle velocity, too.

    Having shot .308's at 100 feet elevation all the way up to 8200 feet in temperatures from 30 to 105 degrees at 600 yards, there's easily a 1.5 MOA spread in elevation settings I had to make to compensate for atmospheric conditions using the same load.
     
  6. skipdavidson

    skipdavidson <strong>SPONSOR</strong>

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    I agree that in most cases the minor errors are negligible. Here's a few pointers: First, pick a cartridge/bullet that is efficient enough for the max range you are interested in. For 500-600 a 308 might be ok, but if you want 800, try a magnum 7mm or 6.5mm.

    Second, validate your trajectory (prove your data) with field shooting. Len has a version of our Gseven ballistics program available on this site that features a simple solver for your true ballistic profile.

    Third, be realistic. BDC's are perfect for hunting, but for Ultra Long Range, you will need to apply MOA corrections from a drop chart based on real time variables. Our new G7 Ballistic Turrets have the BDC and MOA numbers so you can have your cake and eat it too!

    Finally, with new products coming down the line, BDC's are becoming just as accurate as the Old School MOA correction, with all the benefits of quick, simple adjustment and compensation. The RF we start building in June corrects automatically for incline and air density. It calculates a shoot to range in real time based on your BC and MV (BDC turrets or reticles!).

    We train hundreds of guys a year in our Long Range Hunting Courses. I can certify to the ease of learning and application a correct Ballistic Turret will give to Long Range Shooting. By the way, I am very impressed with your "proof" of the errors you will see for different conditions, rather than just relying on forum posts to make your decision.
     
  7. Bart B

    Bart B Well-Known Member

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    One more thing to "measure." The click values your scope has. While they may state 1/4th MOA, you need to measure what your scope really changes point of impact per click.

    Don't shoot a group, move the sights, then shoot another group. You'll need sub 1/4th MOA accuracy to do that. Instead, clamp the scope in a vise with it aligned on a ruler 50 yards away. Then put the reticule on an even inch mark. Move the reticule 10 MOA on its adjustment. Then see if the reticule moved 5 inches on the ruler. If it moved only 4.5 inches or even 5.5, then your scope's got a 10% adjustment error. Imagine what that means at 600 yards.
     
  8. Mikecr

    Mikecr Well-Known Member

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    10moa @ 50yds = 1.047198 x 10 / 2 = 5.236"
    A 10% error would be 0.5236"
    +/- 10% of 5.236 would be 4.71" and 5.76"

    You can set your adjustments true doing this: Reticle Perpendicularity
    Make sure to use a ScopeLevel.
    I'd dial up/down the BDC at laser ranged targets to confirm it's accuracy. Afterall, whomever is building your BDC should be taking care of the ballistics of it all for you. That's what you're paying for.
    Or this: How to Optically Check a Rifle Scope

    Personally, I use electronics and dial in the field. I account for the 'extra' stuff like slope angle, coriolis, and spin drift. But then the kill zone on groundhogs is relatively small, and I give myself only 1sht on any particular GH per day(my rule).
    I think you'll do well with your plan.
     
  9. Bart B

    Bart B Well-Known Member

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    Nope. It all had to do with the competition targets' scoring rings spaced at even inch diameters many long years ago.

    A shooter's minute of angle was established over a century ago with Springfield 1903's 30 inch sight radius and micrometer rear sights moving .008333.... inch per minute or every 4 clicks on 40 tpi lead screws. That's 1/3600th of 30 inches; so is exactly one inch relative to 100 yards. All the Lyman, Vaver, Redfield, Gates, Clerke, Warner and other American made aperture sights were/are made this way. European aperture sights use a metric system that's different than MOA's.

    Furthermore, when externally adjusted scopes were made popular, their adjutments moved exactly .002 inch with 4 clicks and with a 7.2 inch spacing on the mounts, that's also 1/3600ths of the spacing. So they too, moved 1/4 inch per click on the target. Such is life with Unertl, Remington, El Monte, Lyman, Sidle, Litchert and other externally adjusted scopes.

    This is the shooting sports; not trig. There's four different meanings for a "mil" so 'tis no big deal to just learn what it is.

    Col. Hatcher proved in the 1930's that a .30-06 M1 bullet only spin drifts about 3 inches at 1000 yards. And there's not enough coriolis to worry about. I doubt anybody can judge first shot wind drift to that accuracy.
     
    Last edited: May 19, 2011
  10. SBruce

    SBruce Well-Known Member

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    +1

    As a guide, and having a range right here at the house, I have been astonished, and totally amazed by how many scopes don't move the poi the advertised amount per click. Over the years I've watched and helped many many people sight in or check their rifles on my range, and it is unreal how many times I've seen 8 clicks move POI by nearly 3 moa. ViseVersa also, many times I've witnessed a 1 moa correction dialed in, but see no change in POI. Granted most of these scopes are not the top tier $1000-$2000 scopes, but it still amazes me. Seen it soo many times that it almost makes me afraid to dial the knobs!

    I guess my point is; no matter what system we choose to use, it needs to be tested and verified and then maybe even re-verified.

    OP, nice homework on the "within a click":)
     
  11. Mikecr

    Mikecr Well-Known Member

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    Bart, you didn't refer to nicknames for inches per hundred yards (IPHY)(shooter's MOA) earlier.
    But a minute of angle is just that, and not at all a generalization.
    1MOA =(120*3.141592653)/360 = 1.047197551 IPHY
    1MOA is NOT 1.000000 IPHY


    As far as cheap scopes, well, they could be anything really. And IMO, any scopes with capped adjustments at any price, or scopes less than $1K, should be suspect until tested.

    But I've checked many Mk4s and NXS scopes, and I've yet to find an error in moa(actual) adjustments and repeatability. I'm confident that scope makers to this level and above are familiar with MOA, IPHY, and MILs.
    Other than MK4s, many Leupold scopes adjust in IPHY and are 1/4" clicks. It's a good standard(my favorite).
     
  12. SBruce

    SBruce Well-Known Member

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    Mikecr, I actually prefer IPHY too. It just amazes me how many "economy" scopes arent even close to true MOA or IPHY either one!

    Believe it or not, I've even seen this with a couple of the older Leupold Vari-X series.
     
    Last edited: May 19, 2011
  13. davkrat

    davkrat Well-Known Member

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    Absolutely and I have every intention of checking and double checking the data and measuring true MOA needed for each 100 yards out to 600 yards at the 5,000' elevation. I just posted this info to see what the computer models would show. I've always thought it was funny that people argue about being a portion of an inch off at long range but the nature of the scopes coarse adjustment makes it a moot point.
     
  14. DZelenka

    DZelenka Well-Known Member

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    I just checked the CDS dial on my Leupold 3.5-10x. It was nearly perfect for true MOA at 15 MOA above my 100 yard zero.