The Firing Line Forums

Go Back   The Firing Line Forums > The Hide > The Art of the Rifle: General

Reply
 
Thread Tools Search this Thread
Old October 7, 2021, 06:38 AM   #1
tpcollins
Senior Member
 
Join Date: February 18, 2009
Location: SE Michigan
Posts: 555
Question for the MOA aficionados

I realize this isn’t a firearm example but the answer should work the same way.

I have a trajectory that has a 10.17” drop at 50 yards with a 30 yard zero.

If I divide 10.17 by .5 (50 yards / 100 yards) = 20.3 x 1.047 = 21.3 MOA drop . . . yeah or nay?


I assume holdover equals drop but “clicks up” is different. Thanks.




.
__________________
What direction did that last shot at Kennedy come from?

Last edited by tpcollins; October 7, 2021 at 06:44 AM.
tpcollins is offline  
Old October 7, 2021, 07:07 AM   #2
tpcollins
Senior Member
 
Join Date: February 18, 2009
Location: SE Michigan
Posts: 555
Actually I think I should divide by 1.047, not multiply . . .


= 20.3 x 1.047 = 21.3 MOA drop versus 20.3 / 1.047 = 19.39 MOA
__________________
What direction did that last shot at Kennedy come from?
tpcollins is offline  
Old October 7, 2021, 12:59 PM   #3
HiBC
Senior Member
 
Join Date: November 13, 2006
Posts: 7,613
It may involve more variables than you are considering. If you are involved in shooting a firearm,you must consider the line of your sights is offset from the line of the bore.

If you are just working out a geometry problem,make a reasonably nice drawing, Use a straight edge. X-Y grid paper may help. Identify the triangles
and team up with Pythagoras.Its mostly basic trig.

If you go to the Hornady Bullet Website, you can access a free ballistic software to play with. Its an easy to use,good tool
HiBC is offline  
Old October 7, 2021, 03:12 PM   #4
stinkeypete
Senior Member
 
Join Date: July 22, 2010
Location: Madison, Wisconsin
Posts: 925
I will chime in that the science idea of "Unit Cancellation" will help in almost all science problems. Let's pick Inches as our "convenient" unit of measure. (People that live in countries with the metric system are going to laugh in disbelief at what we have to do here.)

50 yards x (3 feet/yard) x (12 inches/foot) = 1800 inches "adjacent side" for the base.
10.17 inches is the "opposite side" of the triangle (You can use -10.17 if you like, too)

Tan(A) = Opposite/Adjacent
Inverse Tangent( Tan(A)) = Inverse Tangent (Opp/Adj)
A = .3237177 DEGREES. (Note: find the inverse tangent of 1. That should = 45 degrees or your calculator is in radian unit mode.)

0.3237177 Degrees x (60 minutes/1 degree) = 19.42306 MINUTES of angle.
The deviation from the point of impact from the laser-sight boreline of the barrel is 19.42306 minutes of angle.

There are 60 minutes of angle per degree, and 60 seconds of angle per angular minute. These wacky units are convenient when doing land surveying and considering longitude and latitude back in the days when one had to do all divisions by hand, and before the decimal number system was widely understood. If you want to divide (30 degrees 40 minutes 20 seconds) in half, it's easy to see it's 15 degrees 20 minutes 10 seconds.

With modern electronics, it's simply obsolete. We forget that modern electronics have only been around for about 50 years. In our hobby, we use other inconvenient units like grains of powder, drams (shotgun shells), ounces (shotgun pellets), minutes, yards, miles.. and I am sure more. History going back a couple of hundred years.

To be exact, a Minute of Angle is close to 1" at 100 yards but it's really 1.047 inches per 100 yards but 8.37" at 800 yards. Except we use fractions of inches (just to drive people crazy) and I'd say ".37 is about .375" which everyone knows is 3/8 of an inch. So at 800 yards the difference is 3/8". Maybe not enough for you to care about, but since we pulled out the calculator anyhow, why not do it right?



This way of doing things doesn't use rough approximations, other than rounding off the degrees to 7 decimal places. the inverse tangent function is various places on various scientific calculators or calculator apps.
__________________
I hunt, shoot bullseye, plink, reload, and tinker with firearms. I have hung out with the Cowboy Action fellas. I have no interest in carrying firearms in urban areas.

Last edited by stinkeypete; October 7, 2021 at 03:29 PM.
stinkeypete is offline  
Old October 7, 2021, 04:13 PM   #5
Rhodester
Member
 
Join Date: January 30, 2019
Location: North Canton, Ohio
Posts: 19
My head hurts.
Rhodester is offline  
Old October 7, 2021, 05:01 PM   #6
stinkeypete
Senior Member
 
Join Date: July 22, 2010
Location: Madison, Wisconsin
Posts: 925
Well, my Honors Geometry students would all be able to do that calculation. It’s basic unit conversion and triangle trig, freshman/sophomore level math.
__________________
I hunt, shoot bullseye, plink, reload, and tinker with firearms. I have hung out with the Cowboy Action fellas. I have no interest in carrying firearms in urban areas.
stinkeypete is offline  
Old October 7, 2021, 07:14 PM   #7
HiBC
Senior Member
 
Join Date: November 13, 2006
Posts: 7,613
In shop math language, you have to convert all the numbers to the same units of measure (inches or feet or cubits,doesn't matter. But you may need your range to the target in inches if you measure your sight radius in inches,etc.)
HiBC is offline  
Old October 7, 2021, 09:51 PM   #8
TXAZ
Senior Member
 
Join Date: September 5, 2010
Location: McMurdo Sound Texas
Posts: 4,308
Quote:
Originally Posted by tpcollins View Post
I realize this isn’t a firearm example but the answer should work the same way.

I have a trajectory that has a 10.17” drop at 50 yards with a 30 yard zero.

If I divide 10.17 by .5 (50 yards / 100 yards) = 20.3 x 1.047 = 21.3 MOA drop . . . yeah or nay?


I assume holdover equals drop but “clicks up” is different. Thanks.




.
Sounds like a high end slingshot at 660 ft/ sec…
__________________

Cave illos in guns et backhoes
TXAZ is offline  
Old October 8, 2021, 11:53 AM   #9
tpcollins
Senior Member
 
Join Date: February 18, 2009
Location: SE Michigan
Posts: 555
I got figured out.

My 2nd post was correct.
__________________
What direction did that last shot at Kennedy come from?
tpcollins is offline  
Old October 9, 2021, 01:07 AM   #10
Pathfinder45
Senior Member
 
Join Date: January 7, 2008
Posts: 3,076
Quote:
Sounds like a high end slingshot at 660 ft/ sec…
Not even that good....more like the slingshot I made when I was 8 using a forked stick and rubber straps cut from an old inner tube.
Pathfinder45 is offline  
Old October 9, 2021, 07:04 AM   #11
std7mag
Senior Member
 
Join Date: June 23, 2013
Location: Central Taxylvania..
Posts: 3,349
Just shoot the freak'n thing already!!!

Took longer to ask the question, and get a response, than to shoot it (whatever it is) and get meaningful field data. Otherwise known as dope.

I never got past first year algebra.
__________________
When our own government declares itself as "tyrannical", where does that leave us??!!
std7mag is offline  
Old October 9, 2021, 01:52 PM   #12
JohnKSa
Staff
 
Join Date: February 12, 2001
Location: DFW Area
Posts: 23,912
The first thing you need to know is what 1 MOA is at the distance in question.

Here's a really simple formula for calculating the value of 1 MOA in inches at any distance given in yards.

Distance (yards) / 95.5 = MOA (inches)

That will give you an answer with an error of less than 0.01%

50 / 95.5 is about 0.524 which means that the value of 1 MOA in inches, at 50 yards is about 0.524"

You want to know what 10.17" at 50 yards works out to in MOA. 10.17" /0.524" is about 19.4 MOA.
__________________
Do you know about the TEXAS State Rifle Association?
JohnKSa is offline  
Old October 11, 2021, 06:50 PM   #13
hounddawg
Senior Member
 
Join Date: March 1, 2009
Posts: 3,986
when I can hold and shoot down to .047 inch accuracy I will sweat it, until then........
__________________
“How do I get to the next level?” Well, you get to the next level by being the first one on the range and the last one to leave.” – Jerry Miculek
hounddawg is offline  
Old October 12, 2021, 12:13 AM   #14
JohnKSa
Staff
 
Join Date: February 12, 2001
Location: DFW Area
Posts: 23,912
In this case, let's say we approximate 1MOA as 0.5" at 50 yards, instead of using its actual value of 0.524"

Then we would calculate that 10.17" was 20.3 MOA instead of the correct value of 19.4 MOA.

If we dialed the 20.3 MOA value into the scope, we would be off by 4 clicks in a scope with 1/4 MOA clicks--half an inch at 50 yards and an inch at 100 yards.

Not a huge error, but definitely a lot more than 0.047".
__________________
Do you know about the TEXAS State Rifle Association?
JohnKSa is offline  
Old October 12, 2021, 04:39 AM   #15
hounddawg
Senior Member
 
Join Date: March 1, 2009
Posts: 3,986
bet if you do this test you will find your scope tracking off by more than .047 inches also

https://www.youtube.com/watch?v=12Wf0Cuwwi8

to quote a previous poster

Just shoot the freak'n thing already!!!
__________________
“How do I get to the next level?” Well, you get to the next level by being the first one on the range and the last one to leave.” – Jerry Miculek
hounddawg is offline  
Old October 12, 2021, 11:55 AM   #16
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 8,677
https://www.nssf.org/shooting/minute-angle-moa/

Most exact number I've seen is this one to a hundred places...

1.04719753642832854694747069666400334739860873986429830552235157457471965151538005004775737357536725837...

.... inches per hundred yards.

However, most bullseye targets in the USA have rings in inch spaced sizes. They are set at ranges measured in yards. Therefore, using MOA values in inches simplifies many issues.

Aperture rear sights made in the USA normally have 40 tpi adjustment screws. With 12 clicks per turn, the standard 30 inch sight radius from the front sight enables 4 clicks to move impact exactly 1 inch per hundred yards.

Last edited by Bart B.; October 12, 2021 at 04:14 PM.
Bart B. is offline  
Old October 12, 2021, 11:40 PM   #17
JohnKSa
Staff
 
Join Date: February 12, 2001
Location: DFW Area
Posts: 23,912
Quote:
Therefore, using MOA values in inches simplifies many issues.
I don't believe anyone is advocating anything else.

For those who are, 1 MOA at 100m is about 2.909cm--or one can round that to 3cm.

Interestingly enough, approximating 1MOA as 3cm at 100m is only in error by about 3% as opposed to the roughly 5% error generated by approximating 1MOA as 1" at 100yds.
__________________
Do you know about the TEXAS State Rifle Association?
JohnKSa is offline  
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 10:25 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2021 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent: thefiringline.com
Page generated in 0.04524 seconds with 8 queries