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A case for BDC Turrets

 
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  #1  
Old 05-18-2011, 12:06 AM
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A case for BDC Turrets

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.

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  #2  
Old 05-18-2011, 01:02 AM
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Re: A case for BDC Turrets

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.
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  #3  
Old 05-18-2011, 08:58 AM
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Re: A case for BDC Turrets

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.

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  #4  
Old 05-18-2011, 09:29 AM
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Re: A case for BDC Turrets

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  
Old 05-19-2011, 05:24 PM
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Re: A case for BDC Turrets

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.
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  #6  
Old 05-19-2011, 07:26 PM
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Re: A case for BDC Turrets

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.
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  #7  
Old 05-19-2011, 07:37 PM
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Re: A case for BDC Turrets

Quote:
Originally Posted by davkrat View Post
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.
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.
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