Using a rifle scope

[Vargus]

I'm trying to wrap my head around this one. So, not having any more proficient method, suppose I go out and purchase a scope whose range turret is not calibrated to anything. How does that work? Is the y-axis reticle deflection using the elevation turret linear, parabolic, sinusoidal?

And if I were to shoot stuff, I need to keep a chart of bullet drop. So, say I zero it at 100 yards and then change the range to 200, there would be some up or down correction I would have to shoot at in order to hit the target?

[Virus]

Originally Posted by Vargus

I'm trying to wrap my head around this one. So, not having any more proficient method, suppose I go out and purchase a scope whose range turret is not calibrated to anything. How does that work? Is the y-axis reticle deflection using the elevation turret linear, parabolic, sinusoidal?

And if I were to shoot stuff, I need to keep a chart of bullet drop. So, say I zero it at 100 yards and then change the range to 200, there would be some up or down correction I would have to shoot at in order to hit the target?

It's called MOA. And yes certain turrets will either move the impact point on the x axis or the y axis, make sure not to cant your scope or your adjustments will be off ;-)

And typically if you are zeroed at 100 yards, you will have to move the impact point up. I've never seen a caliber that rises on the trip from 100 yards to 200 yards.

Allow me to explain in the next 3 posts

[Virus]

Burn Ballistic Solutions ClickCal Public Alpha

ClrHome
:Output (3,1, Click Calculator)
:Output (4,1, Burn Ballistics)
:Output (5,1, Alpha Release) 
:Pause 
:ClrHome
:Lbl M
:Menu( Click Calculator,Click Calculator, A1,Conversions, A2 , About , A3, Exit, EX)
:ClrHome
:Lbl A1
:Menu( Adjustments,1/4 MOA, B1,1/8 MOA, B2 , Custom , B3, Back, M)
:Lbl B1
:Disp Range in yards
:Prompt R
:(((((R*36)*2)*π)/360)/60)→M
:(1/4)→C
:Disp Drop (Inches)
:Prompt D
:((M/(C))→V
:(D/V)→A
:Disp  A
:Clicks
:Pause
:ClrHome
:Goto A1
:Lbl B2
:Disp Range in yards
:Prompt R
:(((((R*36)*2)*π)/360)/60)→M
:(1/8)→C
:Disp Drop (Inches)
:Prompt D
:((M/(C))→V
:(D/V)→A
:Disp  A
:Clicks
:Pause
:ClrHome
:Goto A1
:Lbl B3
:Disp Range in yards
:Prompt R
:(((((R*36)*2)*π)/360)/60)→M
:Disp 1 click  and  100 yd
:Prompt C
:Disp Drop (Inches)
:Prompt D
:((M/(C))→V
:(D/V)→A
:Disp  A
:Clicks
:Pause
:ClrHome
:Goto A1
:Lbl A2
:Menu( Conversions,Yds. to M., C1,In. to Cm., C2 , M. to Yds. , C3, Cm to In.,C4, Back, M)
:Lbl C1
:ClrHome
:Disp How many yards
:Prompt Y
:(Y*0.9144)→M
:Output(4,2, M) 
:Output(5,2, Meters)
:Pause 
:
:ClrHome
:Goto A2
:Lbl C2
:ClrHome
:Disp How many inches
:Prompt I
:(I*2.54)→C
:Output(4,2, C) 
:Output(5,2, CenWebOffters)
:Pause 
:ClrHome
:Goto A2 
:Lbl C3
:ClrHome
:Disp How many meters
:Prompt M
:(M*1.0936133)→Y
:Output(4,2, Y) 
:Output(5,2, Meters)
:Pause 
:
:ClrHome
:Goto A2
:Lbl C4
:ClrHome
:Goto M
:Lbl A3
:Output (3,1, Click Calculator)
:Output (4,1, Version 1.0)
:Output (5,1, Burn Ballistics)
:Disp A different face
:Disp but the words 
:Disp never change....
:Pause 
:ClrHome
:Goto M

This only calculates how many clicks you need at a specified range with a specified drop in inches (I threw a converter in there too for you european folks) It's also alpha and buggy, but the public and full version that takes load data barrel length, wind clock angle, humidity, and all sorts of fun stuff hasn't been released to the public.

[Virus]

Calculating Minute of Angle

To understand sight adjustments on rifles and accuracy of surveying instruments, you need to be able to calculate minute of angle (MOA). MOA is derived from simple geometry. I won't bore you so here is the formula to calculate MOA at a specific range in yards:

(((((r*36)*2)*Pi)/360)/60)=M

r= range in yards

m= MOA

Let's break down the formula:

Where does the 36 come from?

36 is how many inches are in 1 yard. We work with inches because that is typically standard and 1 MOA is almost 1 inch @ 100 yards. Below is the conversion for you:

1 yd. *(3 ft./1 yd.)=3 ft.

3ft * (12 in./1 ft.)=36 in.

Why multiply it by 2?

We multiply the first part of the equation by two because we calculate MOA using the radius of a circle (1/2 of the diameter aka distance across the midesection of the circle)

Why divide by 360?

Because there are 360 degrees in one circle.

Why divide by 60?

Because there are 60 minutes of arc in one degree.

EXAMPLES

What does 4 MOA equal at 850 yards?

Step 1: Recall the MOA formula:

(((((r*36)*2)*Pi)/360)/60)=M

r= range in yards

m= MOA

Step 2: Plug in 850 for r:

(((((850*36)*2)*Pi)/360)/60)=M

Step 3: Solve using order of operations:

(((((850*36)*2)*Pi)/360)/60)=M

((((30600)*2)*Pi)/360)/60)=M

(((6120)*Pi)/360)/60)=M

((192265.4704/360)/60)=M

(534.0707511/60)=M

M=8.901179185 in

Step 4: Recall we wanted to know what 4 MOA was at 850 yards:

 8.901179185*4=35.60471674 inches

It's obvious you would most likely round the answer to two decimal places.

What does 1 MOA equal at 83 yards?

Step 1: Recall the MOA formula:

(((((r*36)*2)*Pi)/360)/60)=M

r= range in yards

m= MOA

Step 2: Plug in 83 for r:

(((((83*36)*2)*Pi)/360)/60)=M

Step 3: Solve using order of operations:

(((((83*36)*2)*Pi)/360)/60)=M

((((2988)*2)*Pi)/360)/60)=M

(((5976)*Pi)/360)/60)=M

((18774.1577/360)/60)=M

(52.15043805/60)=M

M=0.8691739675 in

Here are approximations of MOA at common ranges (100 yard intervals)

1 MOA @ 100 yards = 1 inch 
1 MOA @ 200 yards = 2 inches 
1 MOA @ 300 yards = 3 inches 
1 MOA @ 400 yards = 4 inches 
1 MOA @ 500 yards = 5 inches 
1 MOA @ 600 yards = 6 inches 
1 MOA @ 700 yards = 7 inches 
1 MOA @ 800 yards = 8 inches 
1 MOA @ 900 yards = 9 inches 
1 MOA @ 1000 yards = 10 inches

[Virus]

Scope Adjustments 101

This section is geared more towards weapons use, but depending on the instrument you can still use the same prinicipals.

Adjusting your scope is a vital part shooting at long ranges. It is realy simple once you learn how to calculate MOA.

D/(M*C)=A

D= Drop (inches)

M= MOA at target distance

C= what one click is @ 100 yard on your scope

A= adjustment in clicks

EXAMPLES

We will use the previous examples from "Calculating Minute of Angle".

What does 1 click equal at 83 yards if your scope has 1/4 MOA adjustments @ 100 yards?

Step 1: Calculate MOA

(((((83*36)*2)*Pi)/360)/60)=M

((((2988)*2)*Pi)/360)/60)=M

(((5976)*Pi)/360)/60)=M

((18774.1577/360)/60)=M

(52.15043805/60)=M

M=0.8691739675 in

Step 2: Recall the Click formula

M*C

Step 3: Plug in M and solve:

0.8691739675 in. * (1/4)=0.217275 in.

What does 1 click equal at 850 yards if your scope has 1/8 MOA adjustments @ 100 yards?

Step 1: Calculate MOA

(((((850*36)*2)*Pi)/360)/60)=M

((((30600)*2)*Pi)/360)/60)=M

(((6120)*Pi)/360)/60)=M

((192265.4704/360)/60)=M

(534.0707511/60)=M

M=8.901179185 in

Step 2: Recall the Click formula

M*C

Step 3: Plug in M and solve:

8.901179185 in * (1/8)=1.112647398 in.

Most people round their answers to usable numbers.

[Virus]

Scope Adjustments 102

Once again this is more firearms related, but some survey insturments have adjustments in clicks.

In the previous lessons you have learned how to calculate MOA and what 1 click on your scope equals at a given range. Now you get to put it all together.

Recall the scope click formula:

D/(M*C)=A

D= Drop (inches)

M= MOA at target distance

C= what one click is @ 100 yard on your scope

A= adjustment in clicks

In Scope Adjustments 101 you learned the "M*C" part of the equation.

EXAMPLES

We will use the examples from scope adjustments 101:

You are shooting high by 5 inches at 83 yards. Your scope has 1/4 MOA adjustments at 100 yards. How many clicks do you adjust your scope?

Step 1: Calculate how many inches 1 click equals:

0.8691739675 in. * (1/4)=0.217275 in.

Step 2: plug the answer into the Click formula:

5 in./0.217275 in = 23.01231159 clicks

Step 3: Round the answer to the nearest whole number.

23 clicks

Your bullet will drop 125 inches at 850 yards. Your scope has 1/8 MOA adjustments at 100 yards. How many clicks do you adjust your scope?

Step 1: Calculate how many inches 1 click equals:

8.901179185 in * (1/8)=1.112647398 in.

Step 2: plug the answer into the Click formula:

125 in / 1.112647398= 112.3446657

Step 3: Round the answer to the nearest whole number.

112 clicks

That's it. You now know how to calculate MOA and adjustments for instruments that use clicks.

[Vargus]

Is that program written in TI-BASIC, was it? I should take a stab at writing it in C.

[Virus]

Originally Posted by Vargus

Is that program written in TI-BASIC, was it? I should take a stab at writing it in C.

Yep. TI-BASIC Indeed.

[The Savage]

You writing a Ballistics program?

I tried writing one in C++ a few years ago... Got to about 4 pages of code, realised i was about 300 pages from completion, though "fuck this" and went and used BalCal .

[Vargus]

Originally Posted by The Savage

I tried writing one in C++ a few years ago... Got to about 4 pages of code

Thar's yur problum!

So yea, I checked out the scope I was looking at and the turrets are calibrated for 1/4 MOAl, and by the division of notches, the turrets show MOA directly. Originally this threw me off since I was used to the numbers being 100, 200, etc meters. So now everything makes sense.

Specifically, I was looking at a 4-16x40mm Center Point scope. It has lots of nice features and a great price. The only odd thing is that it has a Mil-dot reticle with MOA turrets, but whatever. Anyone have some experience with that brand? So far all the reviews I've read said they were great scopes.