AR Sponsor
Posted: 6/22/2009 2:01:47 PM EDT
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I am at a crossroads again with my overly active brain. I am shooting 55 Grain XM193, I have a ballistic chart below for ease of use for my question. I need a 100 yd zero for my rifle, it has been raining for 2 weeks here and all I can use right now is a 25 yd indoor range. So here is my question, it may be confusing so hang with me. Keep in mind I am using a MilSpec Front Sight, an ARMS flip up rear sight (#40), and an EOTech 512 mounted on a GG&G Accucam QD System (Which does not raise the sight above the flatop from a stock mount). I have two questions:
1. How do you calculate sight height? 2. My rifle shooting XM193 has a 25 yd zero right now, so at 100 yds with a 25 yd zero it hits roughly 3.05 inches high. Now if I want to zero my rifle for 100 yds, in a 25 yd range, wouldn't the end result have me aiming center mass at 25 yds and actually hitting 3 inches below center mass for a center mass zero at 100 yds? Sorry for the confusion. From my computerized ballistic chart XM193 55 Grain zeroed at 25 yd (no wind): ––All distance drops in inches Sight Height: 1.5 inches Muzzle Velocity: 3250 fps Range - Bullet Drop/Rise 25 yd - -0.01 50 yd - +1.25 75 yd - +2.27 100 yd - +3.05 150 yd - +3.78 200 yd - +3.27 250 yd - +1.35 300 yd - -2.21 350 yd - -7.70 400 yd - -15.42 Again sorry for the confusion. |
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1. I put my rifle sideways on a piece of paper with a line drawn on it, line up the bore with the line when viewing from above. Then look at your scope or iron sight line and make a mark on the paper where the aim point is. Measure the distance.
I suspect your 1.5 sight height is too low. My version of Point Blank defauts to 1.5 when selecting a load, so I've made the mistake myself. 2. Does your program not have a setting to input the zero distance? With your sight height setting of 1.5 you need to be 0.78" low at 25 to be zeroed at 100. With a more realistic sight height of 2.5, you need to be 1.5" low at 25 to be zeroed at 100. |
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Quoted:
Yes my program allows for zero distance. I have it set for 25 due to the fact of only having a 25yd range at my disposal right now. I might try the 50yd zero idea as well, thanks for the input. The Reminton program allows you to set a 50yd zero and tells you how low at 25yds.(~1 1/4"-1 1/2") |
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Ok I updated my ballistic table per what you all have said and have come up with the following:
XM193 55gr 3250 fps Sight Height: 2.5 inches 25yd - 0 50yd - +2.23 75yd - +4.24 100yd - +6.01 So now if I go to the 25yd indoor range, and if I wanted a 100yd zero, would I have to aim center mass at 25yd and hit 6 inches high or 6 inches low for a 100yd zero. |
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Quoted:
Ok I updated my ballistic table per what you all have said and have come up with the following: XM193 55gr 3250 fps Sight Height: 2.5 inches 25yd - 0 50yd - +2.23 75yd - +4.24 100yd - +6.01 So now if I go to the 25yd indoor range, and if I wanted a 100yd zero, would I have to aim center mass at 25yd and hit 6 inches high or 6 inches low for a 100yd zero. Neither. What length barrel do you have? |
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Alright then, for starters, there’s no way that you’re getting 3250 fps from your barrel. That figure is for a 20” barrel. Depending upon a lot of variables, your 16” barrel will sling XM193 at approximately 3100 fps. Next, for an accurate calculation, you’re going to have to measure the distance from the top of your front sight post to the center of your bore line. For the sake of discussion, will use the 2.5” for the “height of sights above bore line” as others have mentioned.
Now for a basic answer to your question; to obtain a 100 yard zero at the expedient distance of 25 yards, you need to adjust your sights so that your point of impact is approximately 1.5” below your point of aim at 25 yards. 
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