Posted: 6/25/2001 7:27:47 PM EDT
OK, I aded a 39x42 scope to my AR15, A1 upper, and I sighted in in at the local indoor range, which is only 25 yards.
I know that one of the newer zero'ing methods is the 50/200 yard method, where you zero at 50 yards instead of meters, and that puts you dead on at 200 yards, and 2" low at 250 yards.
At 25 yards, off a bench, I was getting 5 shot groups about .35" groups, not center to center, but edge to edge at the widest point. The group was 3/8" above point of aim, at 25 yards.
How do I calculate where it would be at 50 yards, so that I can dial it in? It is 1/4/MOA clicks, so 4 clicks would be 1" at 100 yards, meaning that 4 clicks at 25 yards would be .25", do I have this right?
Now, where SHOULD it be hitting at 25 yards, to be zero'd at 50 yards, and am I correct in assuming that I can just dial it in according to what you tell me, and I should be within a click or two at 50/200 yards?
I read some of the recommended methods on some of the AR15 info pages posted throughout this forum, but there are several different methods, and I am not sure which one I matches what I am trying to do.
Help!
please. ;)



Not one person knows? 30 people have read this so far and not one person can chime in with a simple, "5/8" low" or whatever? hehehe
Come on guys, help a brutha out!



For the love of Pete...
The most knowledgeable and heavily visited source of AR15 knowledge in the wrold, and not a single person will post an answer to what should be a simple question...
I am gonna go ask my cat, maybe he knows...



Sorry S. I keep looking every time I see a new post here to get that info myself.
Aviator [img]www.dredgeearthfirst.com/aviator.gif[/img]


One of the fundamental truths of the universe is that there isn't anything that cannot be improved with the addition of pirates, ninjas, midgets or monkeys.

Okay, I'll take a stab. Nothing scientific, just what has always worked for me. I sight in dead on at 25 yds. This gets me on the paper at 100 yds. I then adjust the group center to 2" high at 100. This will be dead on at 50, right on at 200, about 8" low at 300. These numbers hold true for most center fire rifle calibers.223, .3006, etc. You will be surprised how little variation there is from pipsqueak calibers to superboomer magnums. The only way to really KNOW is to shoot at the various ranges.



Finally some answers! hahaha
Not an exact answer yet, but we are getting somewhere... I kept lokking around the net, and one page said that to set your zero to 20 meters, to move it down(I assume) 3/8" from normal, which I am also assuming is a 50/200 zero.
So I moved my scope down 7 clicks. But again, this is all guesswork, I am not sure, as there are so many methods, some in meters, some in yards, etc...
Hopefully someone will stumble in here with an exact number...



Here are 5 methods
[url]http://communities.msn.com/TheMarylandAR15ShootersSite&naventryid=120[/url]



I read the Maryland site, but I could not find the information I was looking for.
I simply wish to know, that if I am going to use the 50/200 yard zero, how low would I be hitting at 25 yards? 1/2"? 3/8"? Is there even a difference from 25 to 50 yards?



You want an exact number from someone, but there are so many variables unknown that no one who knows what they're doing would be able to give you that information.
What's your load's velocity? The ballistic coefficient of your bullet? Distance bore to line of sight? Barrel rise during bullet transit? Temperature? Humidity? And I could rattle off a dozen more. Yes, those are all small factors, but remember, you want an exact number.
If you're on paper at 25 you'll be on paper at 200. If you shoot 200 you'll get your zero then. If you don't, it doesn't matter.



[url]communities.msn.com/TheMarylandAR15ShootersSite&naventryid=153[/url] This covers the 25 Yard method. If your rifle is zeroed at 300 yards then your bullet is rising at 25. Your bullet starts out below line of sight, rises above line of sight, and then fall exactly to line of sight at 300 yards, if thats what you are zeroed at.
25 yards is a very short distance for a rifle zero. I would rezero at a longer distance whenever you do get the chance to.



OK, I realize that I cannot expect an EXACT measurement... hehe
But 25 yards is the limit of the indoor range. That was what I have to work with at this point. So if I wanted to be zero'd at 50/200 yards, I am guessing that I want to be hitting 1/4" or 3/8" low at 25 yards, as the bullet is still on the rise.
Is that approximately correct?



Originally Posted By SquireSCA:
OK, I realize that I cannot expect an EXACT measurement... hehe
But 25 yards is the limit of the indoor range. That was what I have to work with at this point. So if I wanted to be zero'd at 50/200 yards, I am guessing that I want to be hitting 1/4" or 3/8" low at 25 yards, as the bullet is still on the rise.
Is that approximately correct? View Quote 


Be careful about 'clicks' on A2 or is it A3 uppers. One of mine clicks 4 times between the #'s. ie 4 clicks from 200 to 300. The other A3 of mine is actually in half clicks. So it takes 8 clicks to go from 200 to 300.
Make sure you know how yours works when you follow destructions. [;)]


I'm mean nasty&tired. I eat concertina wire&piss napalm and I can put a round through a fleas ass at 200 meters.

Originally Posted By ECS:
Your bullet starts out below line of sight, rises above line of sight, and then fall exactly to line of sight at 300 yards, View Quote 


My range has targets at 20 yards and 33 yards, which should I put my zeroing targets at and what corrections should I made to the windage and elevation for a 25 meter zero?
Kharn



Remember, that I am using a SCOPE, my iron sights are not the issue.
I don't even need to know how many clicks, just need to know that if I want to be zero'd at 50 yards with a 16" carbine, how low(approximately) should I be hitting at 25 yards.
You give me the measurement, and I will figure out the clicks... ;)



Originally Posted By RipMeyer:
Originally Posted By ECS:
Your bullet starts out below line of sight, rises above line of sight, and then fall exactly to line of sight at 300 yards, View Quote View Quote 


Originally Posted By SquireSCA:
Remember, that I am using a SCOPE, my iron sights are not the issue.
I don't even need to know how many clicks, just need to know that if I want to be zero'd at 50 yards with a 16" carbine, how low(approximately) should I be hitting at 25 yards.
You give me the measurement, and I will figure out the clicks... ;) View Quote 


Hi I've been reading along for a while, but had to become a member for this one, as it seemed to be such an interesting question. Here goes a shot at it:
The simple answer is: measure the distance between your bore and your scope and divide it in half. This is the amount you should be low at 25 yards for a 50 yard zero. There are many other factors which play in to this, but at this range and .223 velocities they are neglegible. However, they do form an interesting problem for some quick calculations. The way to visualize ballistics is that you have some sort of sight above your muzzle. The line of sight is ALWAYS a straight line as it is not affected by gravity. The bullet will follow a trajectory as was mentioned earlier. Essentially sighting in is moving your line of sight until is interesects the trajectory. It will do this in two places once with bullet going up, and once as it goes down. For easiers visualization, picture the line of sight being horizontal, and the barrel being angled up to provide the trajectory. If you look at it this way, and say your scope is 2 inches above the muzzle, and you want the bullet to impact on line of sight at 50 yards, it must be half way there at 25 yards, so it will be low 1/2 of the distance between the muzzle and the sight. This is an over simplification, as we will see below, but as shown below, the ignored variables are irrelavent at these distances.


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The above post does not factor in the drop of the bullet as it travels. This drop is given by the formula D= 1/2 a t2. (distance of drop equals one half of the aceleration of gravity times the time of flight squared. The accereration of gravity is a constant 32feet per second squared. Thus we need to know the time of flight to factor in the drop. For this we need the velocity and the ballistic coefficient of the bullet. Lets say you use a bullet with a BC of .34 at a velocity of 3200 fps. The best way to prove this is with a ballistics program, but I don't have one, so numbers are taken from the speer relading manual number 11.
Given a bullet BC of .34 and velocity of 3200, at 100 yds. the velocity is down to 2906 a drop of 294 fps. Now this is where a ballistics table comes in handy, as a good one will calculate the velocity at every foot instead I will interpolate velocities here which will give an approximation of bullet drop. Lets say that at 50 yards the velocity drop is 1/2 of that at 100yds, and the velocity drop at 25 yards is 1/4 of that at 100 yards. This is not entirely true as the velocity drop is not linear (hence why programs calulate it at each foot). Making this assumption, the velocity at 25 yards is 3126.5 fps, and at 50 yds, it is 3053 fps.


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Using the above information, we can approximate the average velocity of the bullet over the flight out to 25 and 50 yards. Again, since the drop in velocity is not linear, we are only approximating. The velocity starts at 3200 fps and is 3126.5fps at 25 yds. This gives an average of 3162.25 (3200 + 3126.5)/2. The average velocity out to 50 yards is 3126.5 fps.
We now use these numbers to calculate the time of flight:
for 25 yds: 75 feet / 3162.25 f/sec = .0237 sec
for 50 yds: 150 f / 3126.5 f/sec = .0479 sec
We can use this to calculate the bullet drop (1/2 at2 formula)
Thus the bullet drop at 25 yds is:
1/2 x 32 f/sec/sec x .0237 squared = .00898 ft
or .10 inches
For 50 yds:
1/2 x 32 f/sec/sec x .0479 squared = .0367 ft
or .44 inches.
Therefore, the differece between the 25 and 50 yd drops is .34 inches, so at this bullet BC and velocity, you would have to add .34 inches to the number you obtained when you divided the distance between your muzzle and sight in half to find the amount the bullet must come up at 25 yards for a 50 yard zero ( i.e. if there is 2 inches between sight and barrel then 1" plus .34", which means you would be 2"1.34" low at 25 yds, or .66". I guess this came out to be more significant than I thought. I think this is right, now if you get a chance to check it in reality, let me know. P.S. don't forget to refigure based on your BC and velocity.


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Hi, I just saw this and I thought I'd post some info & corrections.
First off its a 50 yard and 200 METER zero (it would be about 1" high at 200 yards). This is for the Iron Sights and other optics that are 2.6" over the bore. You scope on TOP of the carryhandle is probably 1" higher. Height of the sights over the bore is part of setting up a zero.
Using irons sights your bullet will strike 1.2" low at 25 yards for the 50/200 zero.
You really should use a ballistic calculator and figure out what works best for you; however you could try zeroing with the bullet striking 2"2.2" low at 25 yards and see how that is for a start.
Shoot Straight,
Forest
Manager of the Maryland AR15 Shooters Site


'98 Jeep XJ Owner
6.8 > 6.5 
BTW this is all a moot point, as I think the 50/200 method is dependent on the height of the iron sights above the bore, so it will not work with a scope on the carry handle. You will be significantly high at 200, as your climb to get to zero at 50 is significantly steeper.


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This has been driving me nuts all morning. I think I made a mistake at the end up there. The drop is 0.44 inches at 50 yds, but only half of that has to be made up at 25 yds, so it could be that you should be 2" (height of sights) + 0.44 (amount of drop) all divided by 2 (half way to 50 yards when the bullet hits at 25 yards) minus 0.1 (drop at 25 yards) which would be 0.78 inches low at 25 yards. But, the more I think, the worse it gets. I guess that's why God made ballistics calculators.


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Damn... I had no idea it was this complicated... hahaha
OK, I will simply sight it in at 100 yards, so that it hits 1" high, which should put me at roughly centered at 200 yards, correct?



Let's make this easy.
WHERE CAN I DOWNLOAD A BALLISTIC CALCULATOR?
hehe



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