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Zeroing scope

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ofar View Drop Down
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    Posted: January/13/2020 at 13:39
I have read a lot about zeroing my scope and in many explanations 100 yards is used as a example distance to zero a rifle scope. And in several discussions/ videos a drawing of the line of site of the scope intersecting with the 'highest' point in the bullet's trajectory at 100 yards (the zero distance) seems to me to be one of two scenarios. Isn't it possible (second scenario) that the line of site of the scope (if gun is stationary and reticle lowered more) could intersect the bullet's trajectory at two points ( A and B)?  'A' being a point along the bullet's ascent (before reaching its highest point along trajectory) and 'B' being a point along the bullet's descent? If so, in the first scenario, the gun would be zeroed at 100 yards, but in the second scenario it would be zeroed at a distance less than 100 yards and another distance farther than 100 yards.   Am I understanding things wrong or am I presenting a valid point? If I'm correct, then the rule that a gun will always shoot 'low' if aimed at a target closer or further than the point of zero (100 yards in many explanations) would not always be true (if the second scenario applies). Right? Any advice would be appreciated.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: January/13/2020 at 13:56
A 100 yard zero is convenient for many reasons, some are:
1. Most ranges have a 100 yard firing line.
2. Most people suck at long range, but 100 is doable for the average shooter.
3. With increased distance comes increased difficulty, and most people hate difficulty.

The bullet does indeed cross the optical line twice, once on ascent and again on descent - in most shooting situations. The USMC used to use a 25m/250m zero with the old M16A2s. I haven't done this with an M4, but do run my M4s with a 36 yard zero, that puts all strikes with the rifle's effective range within a 10" plate - or zone of maximum effect.

All this hinges on the specific round's ballistics and on the shot being taken without significant elevation change from shooter to target.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kickboxer Quote  Post ReplyReply Direct Link To This Post Posted: January/13/2020 at 14:51
I use the 36yard zero, mostly, with M855, which is what I shoot the most of with AR15/5.56.  It's an interesting zero.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Denys Quote  Post ReplyReply Direct Link To This Post Posted: January/13/2020 at 17:27
The issue is one of geometry.  The scope's ling of sight will be essentially parallel to the bore, at about 1.5 to 2.0 inches above the bore.  If you zero the scope at 100 yards, you have to tile the barrel up just a little bit because the bullet starts falling to the ground as soon as it leaves the confines of the bore.  In this case, the angle of bore will be about 2 to 3 minutes of angle up from zero and that will be just enough for the bullet to "rise" up to the 100 yards zero before going back down again.

For instance, in JBM, I did a quick ballistic plot with an MV of 3000FPS, an increment of 10 yards, a range of 200 yards, a zero at 100 yards and a sight height of 2.0 inches above the bore.

These are the numbers I get:  Left is distance in yards, right is inches below the line of sight.
0     -2.0    
10     -1.6    
20     -1.3    
30     -1.0    
40     -0.7    
50     -0.5    
60     -0.3    
70     -0.1    
80     -0.0    
90     0.0    
100     -0.0
110     -0.1

As you can see, the bullet reaches its apogee at 80 yards and then starts going back down after 100 yards.

Now, depending on the height of the sights (the line of sight) above the bore, the barrel can be pointed further up or down to reach that 100 yards.

If the sight height is higher, the apogee of the trajectory will be reached further downrange,  In the case of a height of 3 inches, that occurs at 100 yards and the bullet starts going down after 120 yards.

If the sight is even higher, then the bullet actually continues to rise after the 100 yards zero is reached and in the case of a height of 4.0 inches, the apogee is reached at 120 yards when the bullet is actually 02 inches above the 100 yard zero and remains so for the next 20 yards before going down again.  It actually crosses the light of sight once again at 170yards downrange.  So this is where you have the same zero at two different distances, 100 and 170 yards.

As was discussed already, there are combinations of velocity, sight height, barrel tilt and so on that allow one to use the near zero of the trajectory and know that the far zero will be at a specific distance, much further down range.  For example the 24meter/250meter for the rifle/ammo specified above.

That's a neat way of getting a decent zero at 250 meter, but just shooting at 25 meters.

In my game where the zero is 1000 yards, even with my 2.0 inch sight height, the bullet first crosses the line of sight inside of 8 yards and the actual distance is extremely critical, down to inches, which makes it very difficult to use.  So, I shoot at tall targets at 100 yards and I see how far above the point of aim the bullet hits.  For my load, that's about 35 inches high at 100 yards for a 1000 yard zero.

The neat thing though, is that I can use a boresight laser indoors to aim at 8 yards and confirm the 1000 yard zero.  Of course, not many high-magnification scopes focus down that close, and you have to measure the distance EXACTLY.

I hope this helps.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Denys Quote  Post ReplyReply Direct Link To This Post Posted: January/13/2020 at 17:34
I see we can't edit our posts on this site.

I wanted to add that my 1000 yard trajectory reaches its apogee of 10 FEET 5 inches at 540 yards downrange and starts coming down after that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sparky Quote  Post ReplyReply Direct Link To This Post Posted: January/13/2020 at 21:07
Originally posted by Denys Denys wrote:

I see we can't edit our posts on this site.

I wanted to add that my 1000 yard trajectory reaches its apogee of 10 FEET 5 inches at 540 yards downrange and starts coming down after that.






Once you reach 50 posts then you can edit them.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Denys Quote  Post ReplyReply Direct Link To This Post Posted: January/14/2020 at 07:59
Originally posted by Sparky Sparky wrote:

[QUOTE=Denys]
Once you reach 50 posts then you can edit them.


My mistake.  I thought this forum was for adults.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: January/14/2020 at 09:02
Denys, I appreciate your thoughtful and well written post; however, you are a guest, as are we all, and guests are given limited capacity to interact till they demonstrate they can act like intelligent, civilized individuals.
The forum is for adults, and only one of us is acting like it, as of the last 2 posts.

Welcome, I appreciate the effort that went in to that first post, but things fell off dramatically after that. With more of the prior, less of the latter, you are welcome to stick around.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mike650 Quote  Post ReplyReply Direct Link To This Post Posted: January/14/2020 at 10:34
I think I see what happened...

A known problem on this site among regulars here is when you quote someone the beginning of your reply appears at the far right of the screen, then the rest of the reply shows below the quote. Quoting others used to work but has been broken for sometime now. The fix is when quoting someone, hit the enter key several times from typing. Silly fix, most don't know this.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Urimaginaryfrnd Quote  Post ReplyReply Direct Link To This Post Posted: January/14/2020 at 17:15
https://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi  run a balistic chart for your rifle and ammo to give you an idea what to expect at what distance.   You are correct that while looking from the side at the curve created by the path of the bullet there are two places where the straight line of sight will intersect that path.  With hunting rifles I typically zero at 200 yds. I do that by putting a box out at 25 yds and roughly adjusting the scope then moving it to 50 yds roughly adjusting then to my actual target at 100 yds where I want impact just a couple inches above center X then I go to 200 and fine tune it so its is dead on at 200 yds.  I find most of my hunting shot opportunities are between 150 and 200 yds from a fixed location but sometimes they are closer. What I do can also depend on am I using a musket  then the zero is 100 and the scope had a ballistic reticle which I test at 200. Or  a slower clunky bullet like a 45-70 that has a lot of drop which I zero at 100 and test at 200 to see where my load strikes at that distance.  Generally for hunting if you are zeroed at 200 yds with most deer  calibers like 30-06 you can aim at any deer from close to about 250 and hit vitals.  I will mention a couple things that I find helpful to know about scopes.  In low light dial the power down to around 6x for a brighter image.  Ballistic Reticles are usually designed to work at the maximum power of the scope.
There is no substitute for time on the range with the particular load you are using in your particular rifle because the length of the barrel your elevation above sea level and a lot of other things can make a difference.  Then there is WIND and it will blow the bullet to the side it is not coming from how much is often hard to calculate even for the best shooters.  I have a mix of both 1st and 2nd focal plane scopes the 1st focal plane scopes are typically tactical scopes and as you adjust the power from high to lower the reticle appears to be getting smaller but in fact the marks usually mil dots on the reticle always represent the same thing which means they can be used to determine distance to an object of known size like a 16 in back to brisket deer as well as being used for hold over at known distance.  Not sure why nobody makes a mil dot FFP hunting scope with capped adjustments.  The Super Sniper 3-9x42 could be made as a hunting optic with capped adjustments  and marks added on the reticle like was done with the 3-15 designated marksman model to help estimate distance for 16 in deer and 24 in elk.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Urimaginaryfrnd Quote  Post ReplyReply Direct Link To This Post Posted: January/14/2020 at 17:29
Originally posted by Kickboxer Kickboxer wrote:

I use the 36yard zero, mostly, with M855, which is what I shoot the most of with AR15/5.56.  It's an interesting zero.





I understand that one.   Thought I would attach the explanation for those who don't.
Got to love how this guy has a tourniquet attached to the stock, someone's thinking here.


Edited by Urimaginaryfrnd - January/14/2020 at 17:45

"Always do the right thing, just because it is the right thing to do".
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