How to true the BC

[WahooYahoo]

I use a LabRadar chronograph and I use the mean velocity of as many shots as I can. I trust those numbers. I couldn't trust the numbers from my old wonky Chrony. I wouldn't adjust the velocity.

I also use the Revic BR4 and I true my data based on drops.

BC is fluid, not constant and not linear. The published number is an average. It is a starting point. The BC is based on the test rifle, not mine or yours.

Go to distance (before it goes trans-sonic) and measure the drop, true the BC in your calculator based on that drop. All of the data from bore to that range will be on.

 

[Hondo64d]

I imagine this has been discussed before but a quick search didn't come up with anything recent!

I am using a ZEISS rf10x42 that calculates elevation and atmospheric conditions, into the ballistic calculator. I am always low on my shots past about 600 yards, give or take. A couple weeks back I was shooting at 1250 and was hitting a good 16" low. My muzzle velocity is on per my magneto v3 so I'm thinking the 180 eldm doesn't have quite the BC advertised. I guess the easy way would be to shoot at 900 yards and fidget with the bc calculator until it gives me the same mil correction. Is there a better way? I don't have access to another chrono and the magneto doesn't work downrange.

First things first. Is the scope tracking correctly? Do a tall target test to verify that. If the scope is not tracking correctly, it will skew your results. Make sure your inputs to your ballistic calculator are correct. Scope height, zero offset, atmospherics, etc, really matter.

Im not familiar with Zeiss' ballistic calculator, so I would bounce its computations against JBM, Ballistic AE or another proven ballistics engine to verify its giving you correct data.

Once you've proven none of the above is an issue, if Bryan Litz has provided a BC for the bullet I am using, I trust it and concentrate on truing velocity. Ballistic AE has a velocity truing function which makes it very easy. I seldom use a chronograph anymore. I get a precise 100yard zero, put in an educated guess on velocity in Ballistic AE, and go right to 900 yards, because that's as far as my range goes. I true the velocity at 900 and have never not had the trajectory match when I'm done.

John

 

[Ucsdryder]

Ooookay, so here's my update.

I went out today and did a tall target test. 100 yarfs, 10 mils. I did not tape out 100, I used my rf10x42 binos which I now regret. The far left is the 7saum. 2 shots and then 2 more. As you can see it likes to shoot holes in holes! The right is the 6.5cm. It likes little groups too, I shot 3 shots, adjusted and shot 3 more.

The 6.5 measured at 35 1/16" and the 7saum measured 35 3/16"…goal of 36". I should have taped it off to confirm I was exactly 100 yards, and got rid of any possible variables, but I didn't. So I could have been at 99 or 101 or somewhere else? The fact that BOTH scopes were off by about an inch makes me wonder if my test was off (distance).

Sometimes shooting is fun, other times I feel like all I'm doing is chasing my tail. Right now I'm chasing. (Apparently I do a good job mounting my scope square though!)

Not really sure where to go from here. I think I'll need to re-do the test at exactly 100? Then if it is the scope, what's the solution? Send back? My ZEISS 10x42 RF don't have a way to true up the mils, my only option are scope height and distance. Would changing the 100 yard zero in the computer help? Would it make sense that where it is exactly 36" from zero to 10mph, that distance would true up the computer for long range dope? I assume it would be in the 97-98 yard range.

 

[XLR Industries]

Alright so with this information we know your 7 saum is tracking at about 97.7% and the 6.5 is at 97.3%. I am sure you could call Zeiss and ask them but to some companies, this would be in their acceptable range. Before sending the scopes back you should address if this is going to affect you or not. What is going to be your max range? With pulling some numbers off my phone for my 6.5 PRC at 2700 with the 147 eldm I get these numbers
500 yds- 2.7
700 yds- 4.6
1000 yds- 8.3
1200 yds- 11.5
For this, I will use a 97.5% track and these are what you will actually come up
500-2.6-(.1)
700-4.5-(.1)
1000-8.1-(.2)
1200-11.2-(.3)
If we translate this to inches you are going to be
500- 1.8" low
700- 2.5" low
1000- 7.2" low
1200- 12.9" low
Alright so looking at these numbers, you can ask yourself when does it matter. If you hunt, and shoot out to 700 yards then that is probably not a big deal but if you are shooting at 1200 then it is a really big deal! Is this tracking repeatable too or the next time you do a tall target test is it 103% (1/2 inch high). @COBrad has a scope that tests at like 82% (super old scope). But it is a very repeatable 82% so every time his calculator spits out a number you just do whip your phone out if you have to and get the corrected number. Tracking variance is only an issue if you do not know it is there or if it is not repeatable. If you are aware of it then compensation is super easy!

As far as solutions go, could you zero .1 high so then it cuts your variation at 1000 in half but you are good to go at 700 and in? Is that a better solution for you or are you going to have time to pull your calculator out and do some math? It all comes down to what is going to work best for you in the situations that you are putting yourself in!

Sorry to geek out this hard at 7 in the am but this is what gets me up haha! I love running the numbers for these small factors that can have a huge impact! If you haven't done a tall target test then these numbers should motivate you to do it. I hear a lot of guys talking about how they are comfortable shooting animals at 1000 yards but have no idea of the different compounding variables that go into play at that range! Pretty neat!

 

[Ucsdryder]

Alright so with this information we know your 7 saum is tracking at about 97.7% and the 6.5 is at 97.3%. I am sure you could call Zeiss and ask them but to some companies, this would be in their acceptable range. Before sending the scopes back you should address if this is going to affect you or not. What is going to be your max range? With pulling some numbers off my phone for my 6.5 PRC at 2700 with the 147 eldm I get these numbers
500 yds- 2.7
700 yds- 4.6
1000 yds- 8.3
1200 yds- 11.5
For this, I will use a 97.5% track and these are what you will actually come up
500-2.6-(.1)
700-4.5-(.1)
1000-8.1-(.2)
1200-11.2-(.3)
If we translate this to inches you are going to be
500- 1.8" low
700- 2.5" low
1000- 7.2" low
1200- 12.9" low
Alright so looking at these numbers, you can ask yourself when does it matter. If you hunt, and shoot out to 700 yards then that is probably not a big deal but if you are shooting at 1200 then it is a really big deal! Is this tracking repeatable too or the next time you do a tall target test is it 103% (1/2 inch high). @COBrad has a scope that tests at like 82% (super old scope). But it is a very repeatable 82% so every time his calculator spits out a number you just do whip your phone out if you have to and get the corrected number. Tracking variance is only an issue if you do not know it is there or if it is not repeatable. If you are aware of it then compensation is super easy!

As far as solutions go, could you zero .1 high so then it cuts your variation at 1000 in half but you are good to go at 700 and in? Is that a better solution for you or are you going to have time to pull your calculator out and do some math? It all comes down to what is going to work best for you in the situations that you are putting yourself in!

Sorry to geek out this hard at 7 in the am but this is what gets me up haha! I love running the numbers for these small factors that can have a huge impact! If you haven't done a tall target test then these numbers should motivate you to do it. I hear a lot of guys talking about how they are comfortable shooting animals at 1000 yards but have no idea of the different compounding variables that go into play at that range! Pretty neat!

I'll go through this when I get some time this afternoon and can absorb it all, holy information. THANK YOU!

The scopes are vortex LHT, both identical. I want to shoot out to 1200 and do regularly on steel.

Thanks again for taking the time!

 

[XLR Industries]

I'll go through this when I get some time this afternoon and can absorb it all, holy information. THANK YOU!

The scopes are vortex LHT, both identical. I want to shoot out to 1200 and do regularly on steel.

Thanks again for taking the time!

Lemme know if you have any questions! I love discussing this stuff! To be honest I would be pretty happy with that % of error from an LHT and I would have to bet that is in their acceptable range! Now if it was a Razor, Atacr, or Tangent then that would not be acceptable in my eyes. Kudos to you for testing it too! Most people aren't willing to put the time or ammo into it and just end up scratching their head and changing numbers when dope will not align.

 

[Ucsdryder]

Lemme know if you have any questions! I love discussing this stuff! To be honest I would be pretty happy with that % of error from an LHT and I would have to bet that is in their acceptable range! Now if it was a Razor, Atacr, or Tangent then that would not be acceptable in my eyes. Kudos to you for testing it too! Most people aren't willing to put the time or ammo into it and just end up scratching their head and changing numbers when dope will not align.

Would messing with my bc help? It seems like that might be the ticket, by lowering BC to match the dope at 1000, it seems like that would get me very close. Or am I overlooking something by doing that? When I shot a couple days ago after the tall target I set my g7 bc to 3.2 (3.57) is the book value and had a first round hit at 835 on a milk jug. I'd need to confirm dope from 200-1200 to build confidence. I just know myself and "close enough" isn't going to get it done. If my ZEISS rf10x42 don't have a way to true up the mils, my only other option would be to swap scopes and I hate to do that.

 

[nksmfamjp]

View attachment 369356

What book is that again?

Also, i true first with zero or zero offset. A 100 yd zero off by 1/4" can look rock solid on paper. I get that to give me good hits to about 600 yds. I'm even willing to be a little high or low.

Then I play around with BC or really more jointly with zero until I get the best data I can. Many guys adjust MV, but I consider it a known value. BC shouldn't be too far off though. Neither should MV. I found zero offset the only way to get enough change to get what I needed.

 

[338 dude]

What book is that again?

Also, i true first with zero or zero offset. A 100 yd zero off by 1/4" can look rock solid on paper. I get that to give me good hits to about 600 yds. I'm even willing to be a little high or low.

Then I play around with BC or really more jointly with zero until I get the best data I can. Many guys adjust MV, but I consider it a known value. BC shouldn't be too far off though. Neither should MV. I found zero offset the only way to get enough change to get what I needed.

 

[shootnpoke]

Just a stupid idea and question for all here. OP states he shot a tall target test @100 yds with 10 mil elevation. Would it not be better to zero and try the tall target tests at 100 meters with a mil scope?

 

[nksmfamjp]

Just a stupid idea and question for all here. OP states he shot a tall target test @100 yds with 10 mil elevation. Would it not be better to zero and try the tall target tests at 100 meters with a mil scope?

Why does 100m make a difference? You know mil is not metric right?

1. In the International System of Units, the derived unit of planeangularmeasure of angle equal to the angle subtended at the centre of a circle by an arc of its circumference equal in length to the radius of the circle. Symbol: rad

Apparently I am wrong. It is an SI unit. My point was that it is a ratio defined as 1m/1000m or 1yd/1000yds. That is why I said it is not metric. Since it is defined by SI, that makes it metric.

 

[XLR Industries]

Would messing with my bc help? It seems like that might be the ticket, by lowering BC to match the dope at 1000, it seems like that would get me very close. Or am I overlooking something by doing that? When I shot a couple days ago after the tall target I set my g7 bc to 3.2 (3.57) is the book value and had a first round hit at 835 on a milk jug. I'd need to confirm dope from 200-1200 to build confidence. I just know myself and "close enough" isn't going to get it done. If my ZEISS rf10x42 don't have a way to true up the mils, my only other option would be to swap scopes and I hate to do that.

I actually think you could get data to align with adjusting BC! Even though it is not really trueing up data to what it actually is, you could be trueing it up to your system. In this instance, the BC that you put in the calculator would be lower than what the "true" BC would be if your scope is tracking correctly. Since you are going to be fudging the numbers you might get a little bit of variance but I am sure you can align it pretty well! If you have some large steel you can throw it on there and see your impact which is what I normally do when trueing. You could also put it on paper and drive out if you want to get a little finer results. I think you are on the right track though!

Just a stupid idea and question for all here. OP states he shot a tall target test @100 yds with 10 mil elevation. Would it not be better to zero and try the tall target tests at 100 meters with a mil scope?

At 100 yards, a 0.1 mil click is 0.36 inch, and a full mil is 3.6 inches (Practically speaking, 1⁄10 of a mil equals 1 centimeter at 100 meters).

So with that math, 10 mils would be 39.37 inches at 100 meters.

Long Range Shooting - MOA and MILS explained

A MIL or an MOA linear equivalent increases proportionally with distance. With the MOA and MIL systems, you correct the position of the reticle inside the scope, based on wind deflection and trajectory of the bullet at the target’s distance.

www.longrangeshooting.org

 

[shootnpoke]

Ok gents, my bad on the math. I only have 1 mil scope, an old S&B. Pretty sure adjustments on it were set at 100 meters. I apologize for my inability to sipher.

 

[XLR Industries]

Ok gents, my bad on the math. I only have 1 mil scope, an old S&B. Pretty sure adjustments on it were set at 100 meters. I apologize for my inability to sipher.

Your scope wouldn't have been set up a different way you just thought in a different measurement. If you are thinking in meters then you could have every .1 mil is 1 cm at 100 meters. It is the same angular measurement as every click is .36 inches at 100 yards. Just a different way to think about it. The only time that I ever think of it that way is if I am zeroing and do not have a good reticle. Otherwise, I just measure with the reticle and come over/up what the reticle tells me. This is where I got very confused and I feel like a lot of guys do because they are trying to convert it to inches. I still have no idea how many cm go into an inch but it is really irrelevant when shooting because you simply want to measure your poi in mils and make the correction that way.

I am sure this sounds confusing and I will tell you that I was so confused when going from MOA to Mils! I just had to train myself to think in mils and not try to convert it to a stagnant unit of measurement that changes as distance changes. Use what your ballistic calculator spits out for elevation and for wind, use the wind formula that I have mentioned in previous posts to get the angular measurement in mils. Otherwise, stuff gets really confusing!