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Could someone explain bullet drop and BDC reticles to me?

 
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  #1  
Old 05-28-2005, 07:41 PM
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Could someone explain bullet drop and BDC reticles to me?

OK, I'm trying to understand a few things about bullet drop and reticles.

Let's say the average 55 grain FMJ bullet fired out of a 5.56 barrel (.223) 16" M-4 has the following bullet drop (this assumes sighting in at 100 yards):

-3.3" @ 200yds
-13" @ 300yds
-32" @ 400yds
-62" @ 500yds


Now, how much "hold over" is required at 100 yards to hit the target at 200, 300, 400, and 500 yards?

Surely you don't have to aim 62 inches higher from your 100 yard mark to hit at 500 yards, do you?

I recall looking at the Burris Ballistic Plex reticle chart here:


http://www.burrisoptics.com/reticles.html

At the bottom it shows the reticle with standard corrections at 100 yards of 1.5" at 200, 4.5" at 300, 7.5" at 400, and 11" at 500 yards.

Is this more in line with what you would need to do regarding holdover for a 55 grain FMJ bullet fired from a 16" M-4 barrel to be "spot on" at these various yardages?
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  #2  
Old 05-28-2005, 08:27 PM
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Re: Could someone explain bullet drop and BDC reticles to me?

grey
The -3.3", -13", -32", and the -62" are the actual drop of the bullet. If you are sighted in at 100 yds and shoot 500 yards you will need to hold over 62" or dial in 15.5 moa.

I think the chart at the bottom of your link is the correction at 100 yds with a 100 yard zero. For example if you use the second dot down from the crosshair on a 100 yd target you will hit 4.5" high. third dot 7.5" high and top of the bottom post 11" high.
I think, I'm a dial it in guy , not real savy with mil-dots or BDCs.
Chris
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  #3  
Old 05-28-2005, 08:51 PM
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Re: Could someone explain bullet drop and BDC reticles to me?

Grey2112,

This can can a bit scientific but here goes. First the concept of what "Minute Of Angle" MOA is and how it affects the bullet's dispersion pattern as ranges are increased must be understood. I found this on a disk website and I believe it is close and can be helpful.

[ QUOTE ]
True Minute Of Angle, MOA, is measured by first determining the minutes of angle in a circle. We know that a circle contains 360 degrees and that there are 60 minutes in each degree. The minutes of angle in a circle is 360 x 60 or 21,600. The circumference of a circle is 2 x p x R where R is the radius of the circle in inches. Hence, for any range R, a minute of angle equals 2 x 3.1416 x R divided by 21,600 or .000291 x R.

Therefore, if the range R is 100 yards, a minute of angle would be .000291 x 100 x 36 or 1.0476 inches. Because the distance covered at 100 yards is so close to being exactly 1 inch, the fraction is usually disregarded for range work, and we commonly say that a minute of change on the rear sight will move the point of impact 1-inch at 100 yards. If we were shooting at 1,000 yards, a minute of angle would cover .000291 x 1000 x 36 or 10.476 inches. The difference between the true minutes of angle and the rounded minutes of angle becomes greater as the range increases.

[/ QUOTE ]

So basicly 1 MOA equals = 1.045 inches at 100 yards, and increases as the range increases. Most round it to 1 inch - 1 MOA just to ease the process. And except at the "benchrest or at extremely long ranges" the 1" works fine. At least for me anyway. Examples: 200 yards = 2 MOA, 300 yards = 3 MOA... so forth and so on. 500 yards is 5 minute of angle MOA.

To find the theoretical increase in elevation at 100 yards to hit line of sight - point of impact from 200 to 500 yards, divide the range by "100" and you get the MOA for that range. 500 yds is 500/100=5 MOA, 450/100= 4.5 MOA, 475/100=4.75 MOA etc etc. Just think of MOA as one inch, it makes it easier.

If your bullet drop at 500 yards is 62", then divide 62" by 5 MOA and you get 12.4" or 62"/5MOA=12.4". Given this ballistics input(AKA: Dope), if you zero your sight at 100 yards to impact 12.4" high, you theorectically should hit point of impact - line of sight at that range.

That is what the reticle takes into account in the scope. You wrote,

[ QUOTE ]
At the bottom it shows the reticle with standard corrections at 100 yards of 1.5" at 200, 4.5" at 300, 7.5" at 400, and 11" at 500 yards.

[/ QUOTE ]

I compute similar data below manually.

Range - Bullet Drop - MOA
100 - zero point of impact - line of sight
200 - -3.3" 1.65" below line of sight
300 - -13" 4.3" below line of sight
400 - -32" 8" below line of sight
500 - -62" 12.4" below line of sight

In order to correct for the bullet drop at the farther ranges, simply zero your weapon at 100 yards, then crank the scope up the appropriate amount of MOA's or inches from above chart, and you can reasonably be expected to hit at the given farther ranges.

The plex reticle you mentioned does this for you. You can see that the plex formula you provided above, does basicly the same thing that I have done manually in my chart.

I don't like using holdover, mildots or stadias, I much prefer what we call, "cranking in the ballistics dope" to achieve a "line of sight - point of impact" hit. That way, my reticle cross hair never leaves the desired point of impact.

You can also use the same method for determining the exact hold off for wind drift. Stadias and mil-dots can also be set-up to provide for that as well. But that is another thread.

The Bullet Drop Compensator BDC, does the same thing as above. It is pre-adjusted, geared I believe, and measured for the ballistic characteristics of a given round. All you need to do is estimate the range... crank to that range on the elevation turret, hold center and squeeze. Given a no wind situation of course.

Finally, what ever ballistics dope you use, or method of acquiring and then applying that dope for the given load... you must then go to that "given" range, to confirm that it is actually right for your set-up. Then adjust, record for later use, and then practice, practice!

Know this: temperature, humidity, altitude, and even the distance to target all serve to change this dope in day to day varying conditions. I suggest that you keep a solid accurate record of your results each and every range visit, recording those variables. Then you will have built a solid mechanism for returning to the "line of sight - point of impact" desired under varying conditions and ranges.

I know this was wordy, and hope it is helpful.

Many more here probably have a better way of explaining this than I. Good luck.

v/r

CG
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  #4  
Old 05-29-2005, 12:38 AM
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Join Date: May 2005
Location: Missoula, MT USA
Posts: 278
Re: Could someone explain bullet drop and BDC reticles to me?

I always looked at drop compensating scopes like the Shepard and others and I was like "how does it work on different rounds?" How can it be that the additional crosshairs are right on at say 500 yards? Well, I still really haven't figured it out, but from what I understand a BDC scope is preset with the ballistics that are pretty common for each round. I really don't like all that junk that I would have to look through if I had one. They say they are "rangefinding" but the circles or bars with which you are supposed to bracket the brisket are set for eighteen inches or whatever it is. What if you're looking at a calf elk instead of a cow, or an exceptional deer that is bigger? You're supposed to calculate range with it? I hunt and am just getting into this longer range stuff so I am used to holding over my target depending on how far away it is. The BDC scope is meant so you're holding your primary crosshairs over, but you have some more crosshairs in there that are the ones you want to use for farther shots. Hope this doesn't confuse you more than it helps????
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  #5  
Old 06-07-2005, 12:16 AM
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Join Date: Jul 2002
Location: Michigan
Posts: 647
Re: Could someone explain bullet drop and BDC reticles to me?

Regardless of the range finding retical; I always use the Laser ranger finder to range targets.

I have Mildot, Shepherd, Burris Ballistic plex, Redfield accutrac and Horus vision scopes all with specialized reticals for Bullet drop compensation. All work fairly well but you must know the distance using a laser range finder.

I have used the BDC reticals out to 600 meters with deadly accuracy, the HV retical out to 1000, but if I were only to have one scope and style retical I would get a good scope like, Leupold, Xotic Optics, Nikon, Night force or on the lower end Super sniper and Sightron and get a BDC elevation and windage turrets with data built in for my round that I shoot and maybe 2 or 3 different ones for different temperature and elevations. You can have them made at a decent price.

So now all I have to do is range- dial and shoot, the mildots would be use for wind correction on the fly.
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  #6  
Old 06-07-2005, 02:09 AM
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Join Date: Feb 2003
Location: Pueblo, CO
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Re: Could someone explain bullet drop and BDC reticles to me?

1963 definitely hit it all right on the nose-- very succinctly too considering the broad range of this topic.

The last several years spent researching these "tactical" systems in the pursuit of game led me to writing this article on the subject of reticle applications--

www.ottllc.com/specialtypistols/sp20.pdf

If u take the time to study it some it should help. The whole idea of applying reticles for downrange zeroing/windage/ranging can be somewhat complicated UNTIL u begin to realize that it is simply based upon an angular measuring device (reticle stadia) that is used for measuring the 3 aforementioned systems downrange. The more u work with it the more u'll understand it, and the more u begin to understand "the system" the more u'll realize it's really an enlightenment, of sorts.
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  #7  
Old 10-29-2013, 10:00 PM
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Join Date: Oct 2013
Posts: 1
Re: Could someone explain bullet drop and BDC reticles to me?

Quote:
Originally Posted by ceg1963 View Post
Grey2112,

This can can a bit scientific but here goes. First the concept of what "Minute Of Angle" MOA is and how it affects the bullet's dispersion pattern as ranges are increased must be understood. I found this on a disk website and I believe it is close and can be helpful.

[ QUOTE ]
True Minute Of Angle, MOA, is measured by first determining the minutes of angle in a circle. We know that a circle contains 360 degrees and that there are 60 minutes in each degree. The minutes of angle in a circle is 360 x 60 or 21,600. The circumference of a circle is 2 x p x R where R is the radius of the circle in inches. Hence, for any range R, a minute of angle equals 2 x 3.1416 x R divided by 21,600 or .000291 x R.

Therefore, if the range R is 100 yards, a minute of angle would be .000291 x 100 x 36 or 1.0476 inches. Because the distance covered at 100 yards is so close to being exactly 1 inch, the fraction is usually disregarded for range work, and we commonly say that a minute of change on the rear sight will move the point of impact 1-inch at 100 yards. If we were shooting at 1,000 yards, a minute of angle would cover .000291 x 1000 x 36 or 10.476 inches. The difference between the true minutes of angle and the rounded minutes of angle becomes greater as the range increases.

[/ QUOTE ]

So basicly 1 MOA equals = 1.045 inches at 100 yards, and increases as the range increases. Most round it to 1 inch - 1 MOA just to ease the process. And except at the "benchrest or at extremely long ranges" the 1" works fine. At least for me anyway. Examples: 200 yards = 2 MOA, 300 yards = 3 MOA... so forth and so on. 500 yards is 5 minute of angle MOA.

To find the theoretical increase in elevation at 100 yards to hit line of sight - point of impact from 200 to 500 yards, divide the range by "100" and you get the MOA for that range. 500 yds is 500/100=5 MOA, 450/100= 4.5 MOA, 475/100=4.75 MOA etc etc. Just think of MOA as one inch, it makes it easier.

If your bullet drop at 500 yards is 62", then divide 62" by 5 MOA and you get 12.4" or 62"/5MOA=12.4". Given this ballistics input(AKA: Dope), if you zero your sight at 100 yards to impact 12.4" high, you theorectically should hit point of impact - line of sight at that range.

That is what the reticle takes into account in the scope. You wrote,

[ QUOTE ]
At the bottom it shows the reticle with standard corrections at 100 yards of 1.5" at 200, 4.5" at 300, 7.5" at 400, and 11" at 500 yards.

[/ QUOTE ]

I compute similar data below manually.

Range - Bullet Drop - MOA
100 - zero point of impact - line of sight
200 - -3.3" 1.65" below line of sight
300 - -13" 4.3" below line of sight
400 - -32" 8" below line of sight
500 - -62" 12.4" below line of sight

In order to correct for the bullet drop at the farther ranges, simply zero your weapon at 100 yards, then crank the scope up the appropriate amount of MOA's or inches from above chart, and you can reasonably be expected to hit at the given farther ranges.

The plex reticle you mentioned does this for you. You can see that the plex formula you provided above, does basicly the same thing that I have done manually in my chart.

I don't like using holdover, mildots or stadias, I much prefer what we call, "cranking in the ballistics dope" to achieve a "line of sight - point of impact" hit. That way, my reticle cross hair never leaves the desired point of impact.

You can also use the same method for determining the exact hold off for wind drift. Stadias and mil-dots can also be set-up to provide for that as well. But that is another thread.

The Bullet Drop Compensator BDC, does the same thing as above. It is pre-adjusted, geared I believe, and measured for the ballistic characteristics of a given round. All you need to do is estimate the range... crank to that range on the elevation turret, hold center and squeeze. Given a no wind situation of course.

Finally, what ever ballistics dope you use, or method of acquiring and then applying that dope for the given load... you must then go to that "given" range, to confirm that it is actually right for your set-up. Then adjust, record for later use, and then practice, practice!

Know this: temperature, humidity, altitude, and even the distance to target all serve to change this dope in day to day varying conditions. I suggest that you keep a solid accurate record of your results each and every range visit, recording those variables. Then you will have built a solid mechanism for returning to the "line of sight - point of impact" desired under varying conditions and ranges.

I know this was wordy, and hope it is helpful.

Many more here probably have a better way of explaining this than I. Good luck.

v/r

CG

several years later i would like to thank you very much for this explanation! i have wondered that forever and no one really explained it in a way i could understand!
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