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Holding over compared to dialing in MOA

 
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  #1  
Old 02-04-2004, 12:39 AM
 
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Holding over compared to dialing in MOA

This is a partial email that was sent to me from a friend of mine who is in the ballistics’ end of the business. I found what he had to say interesting and wanted to share it with you.


No one seems to grasp this concept except for B. R. and the ballisticians I have consulted with! I have verified everything and beat this thing to death in the past but it keeps rearing it's ugly head! Inches and MOA do not work the same over distance. Inches of drop for 350 yd, when used to hold over on a 500 yd target (45 degree angle) does not have the same value as the MOA for 350 yards when it gets out to 500 yd. MOA is an angular measurement and simply put, the MOA value in inches grows with distance.

I just don't know how else to say it! Most tactical types with experience all dial-in, so it's not an issue. However, I talk to the less informed daily too and I ask many of them how they apply the data. A few of the tactical newbies just starting out want to use hold over only for all shots and don't want to touch their dials. And most of the hunters definitely only want to hold over for all their shots. They often don't even have target knobs on their scopes.

People deserve to know the truth of the limitation of the equipment we offer and I always tell it like it is.

Exbal uses current drag/ballistic tables etc… What are your feelings about this?
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  #2  
Old 02-04-2004, 04:44 AM
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Location: Palmer, Alaska
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Re: Holding over compared to dialing in MOA

W,
First, have you made any angle indicators with the "correction factors" of
(1 - cosine), instead of the cosine or angle, or would you make one?

I've found that adding a flat 4 MOA to what ever my come-up MOA is for an angled shot, then multiplying it by the correction factor to see how many MOA to "take off" the level fire come-up MOA is way easier to remember the actual bore line drop this way. Saves another step in the field if I could look at the angle indicator and instantly see that the 35 degree correction factor is .181 instead of the .819 cosine on the cosine/angle indicator. Just one less math problem to solve, subtracting .819 from 1 to get the .181 I'm looking for.

Okay, I think that 1 inch being so close to 1 MOA both helps, and also confuses many people.

I'm not the best at explaining things, even things I understand well. I was confused by this when I got started figuring drops and corrections to dial, but it's pretty simple really.

I look at MOA and inches when I'm looking at compensating for a groups measured drop in inches on a target at a specific range, mainly. I really don't think in terms of inches, but rather MOA in most any other situation. In other words, I look at MOA on a chart, note everything in MOA, wind drift, on and on and on... Why think in terms of inches when you don't have to and just complicate things... It's like looking at things in terms of mils, but dialing in MOA, something I've never understood...

To me, if 1 MOA was equal to 1", that would really have people confused even more.

My turrets compensate for 1.047" (1 MOA) at 100 yards with every 4 clicks, or number on the turret. If I'm 8" low at 450 yards and want to know the correction to bring it up, first things first - Every 1 MOA I dial, the turret will move the crosshair 1.047" on the target image "at 100 yards", and if the target image is at 450 yards, 4.5 times farther, the crosshair moves 4.5 x 1.047" = 4.71". The crosshair moves lower when you "dial" for the bullet to impact "up". The crosshair moving lower forces you to raise the muzzle to bring the crosshair back up to the target, the more you dial, the more you raise the muzzle.

To calculate the correction for a specific bullet drop at a specific range, you must first understand that 100 yards is your base line, but not the multiple you'll be dealing with in the calculations. If the range is 678 yards away, 6.78 X 1.047 = the number of inches that 1 MOA dialed, or held over, will change the point of impact (POI). In this case, 1.047" at 100 yds (1 MOA) is worth 6.78 times times that, "at this range". Because it's 6.78 times as far, the 1.047" grows at the rate of 6.78 times also.

So, if you're 8" low at 450 yards, you divide the 8" by 4.50, then by 1.047 for the 1.70 MOA solution.

1.70 MOA = (8/4.50)/1.047

8/4.50 = 1.78
1.78/1.047 = 1.70 MOA

Another example -
If in the conditions you're shooting in, you know the bullet drops 109" at 815 yards from your 100 yard zero, (109/8.15)/1.047 = 12.77 MOA is what you dial to rezero for the shot.

Just remember, this is minute of angle not inches. When you dial up 1 MOA of elevation into your scope, you will be raising your bore line (BL) above your line of sight (LOS) 1.047" at 100 yards, but that BL is straight and extends to infinity at that angle relative to your LOS, and the divergance, or distance between this extended BL and LOS will be double at 200 yards (1.047" X 2.00), and triple at 300 yards (1.047" X 3.00) etc, etc... The distance between the LOS and BL, in inches, will be equal to the bullet drop from your 100 yards zero when this drop is correctly compensated for.

1 MOA equals 1.047" of divergance at 100 yards, range multiplies this as described. Divide the drop in inches at range by the range multiple, then by 1.047" to back into the MOA correction needed.

.....

This load I've been working on and figuring angles with for a while now seems to be fairly consistant in one way that will speed up angle calculations some, or save me from having to carry a "drop from BL" MOA chart. This way I can work from my Come-Up MOA only, but retain the precision you only get from using BL drop in the calculations to find a corrected firing solution for angled shots.

My Come-Up MOA at whatever range is always 4.0 MOA less than my level fire BL drop MOA with this scope height, so adding that 4.0 MOA to my level fire range Come-Up, it yields the BL drop MOA I need in order to work the formula for a corrected come-up.

If you look at your BL drop in inches and convert it to MOA, then the same for your bullet path come-ups, subtracting the come-up MOA from the BL drop MOA at each 100 yard increment on out to 1500 yards, you'll see the difference between them at each interval the whole way out. With this 1.75" scope height of mine zeroed at 100 yards, mine runs an MOA difference of about 3.9 - 4.0 MOA all the way on out.

So, the formula to use with this now is, level fire come-up MOA + 4 MOA = BL drop MOA * (1 - cosine) for the correction MOA to go back and subtract from your level fire come-up, for a corrected come-up for that slope range.

Example -
Lased 900 yard shot, 45 degree angle:

Come-Up for Level fire 900 yds = 31.17 MOA

Corrected for angle come-up = 31.17 - ((31.17 + 4.0) * (1 - .707))

31.17 MOA + 4.0 MOA = 35.17 MOA BL drop

1 - .707 cosine 45 deg = .293 correction factor

35.17 MOA * .293 = 10.30 MOA correction

31.17 MOA - 10.30 MOA = 20.87 MOA corrected come-up for 900 yards, 45 degrees.

.....

W,
You can see how a "correction factor" angle indicator would save another small step in the calc's there.

Multiplying the 900 yard come-up (31.17 MOA)by the cosine (.707) gives a 22.03 MOA angle fire solution, and that's a 9.5" - 12" miss depending on which way you dialed using the other method, which is much more accurate.

If guys want to use holdover, the NF R2 reticle is at least calibrated in MOA, not mils, and that's what I'd use, and do. [img]images/icons/smile.gif[/img]

The Exbal program is nice, I use it as well, but it needs G5 and G7 drag curves to work at its fullest. It has some nice new features on the latest versions, the latest Palm version is really nice.

[ 02-04-2004: Message edited by: Brent ]
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  #3  
Old 02-04-2004, 06:16 AM
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Join Date: May 2001
Posts: 2,369
Re: Holding over compared to dialing in MOA

Damn Brent

You posted a book's worth of info. [img]images/icons/smile.gif[/img]


W

Like Brent said somewhere in there..

There are two different types of holdovers, one uses the reticle features (MilDot 3.44 MOA, or NP R2, or other etched markings) and this makes holdovers angular and is fairly easy and exact.

The other method enjoyed by many (not by me though) is trying to calculate the inches or feet or yards of holdover required by judging the correct value at the target and aiming at an imaginary "spot" somewhere above the target... "That looks like 8.23 feet just there at that limb over the elk's head, I'll hold there".... sort of iffy.
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Old 02-04-2004, 08:09 AM
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Join Date: May 2001
Location: Pennsylvania
Posts: 1,757
Re: Holding over compared to dialing in MOA

If one hunts longrange with a partner who has a good set of bigeyes, the shooter simply takes a sighter shot "first" (away from the animal) dial to the bullet impact and then swing back on the animal for the second shot. It doesn't matter if your shooting uphill, downhill or straight across the mountain. That sighter shot eleminates any guess work and all the math and also gives you the indication of what the wind is doing so you can correct.

This way theres no holdover for elevation or wind and you hold the cross hair dead on the shoulder or area you want to hit "after" you have dialed in.

This is probably the most used way the long and ultra longrange hunters do it in this part of the country and when we go west.

I've tried the holdover scopes and don't like them. The animal always seems to be "between" the two dots or two lines in a holdover scope. I like to hold dead on the area I want to hit.

To each his own on this one. Everyone has their favorite.

Mine will always be the dial in method either for 1000 yard benchrest or Longrange hunting. Has worked for me.

Later
DC [img]images/icons/smile.gif[/img]
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Old 02-04-2004, 08:57 AM
CAM CAM is offline
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Re: Holding over compared to dialing in MOA

Brent
I hope you keep all this info saved so you can repost with out retyping!! [img]images/icons/smile.gif[/img]

CAM
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  #6  
Old 02-04-2004, 09:30 AM
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Join Date: Nov 2002
Location: St. Louis
Posts: 189
Re: Holding over compared to dialing in MOA

W,

I'm not quite sure what you're getting at there - but if you want to understand using a graduated reticle for holdover, check out this thread:
http://www.longrangehunting.com/cgi-...c&f=2&t=000584

We've beaten this topic to death, raised it back from the dead, beaten it again, etc...

I and a number of comrades are proponents of graduated reticles for holdover. Check out that thread, and if you're interested, search the site for "R2". You'll get a brain-full of opinions...
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  #7  
Old 02-04-2004, 09:47 AM
 
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Re: Holding over compared to dialing in MOA

I have worked off of mil holds for many years and could not quite grasp what this quote actually meant, until now. I think what he actually was stating was that when shooting on an angle, MOA and MIL Radian values grow with distance and that working off of what you think your inches of drop is does not. I was actually locked up over what he was trying to say.
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