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Posted: 11/13/2002 1:39:52 PM EDT
I am a newbie and I am very confused after going to the Maryland AR15 Shooter Site. They have a link that describes how to zero in a telescopic site equipped with a BDC (bullet drop compensator). The article writer is using a Leaper 4x20 scope, my rifle has the similar 3-9 X 28 Leapers sight, so the procedures are applicable. Heres why I am totally confused:

The instructions are to set a target at 25 yards, and then set the BDC for "4", which is 400 yards. Why???? My scope has 6 position, from zero to 5, corresponding to the distance to target in 100 x yards. Why would I set it to 400 yards when the target is only 25 yards away?

The instruction go on to say to then set another target at 100 yards, and the BDC to "1", which is 100 yards. Now this part makes sense, a "1" setting for 100 yards, yes no prob. I see that.

But why in the heck am I to set the BDC for 400 yards when shooting a target 25 yards away???

Okay where am I going wrong here?
Link Posted: 11/13/2002 2:32:04 PM EDT
[Last Edit: 11/13/2002 2:34:56 PM EDT by Forest]
Marcus,
No worries mate, I'm here to help.

You see bullets are not laser beams they don't follow a straight line. They have a curved trajectory (you actually fire them at an upward angle and gravity pulls them back down).

The bullet will start out from your scoped rifle some 3.5" below your line of sight (which is laser beam straight). If zeroed properly it will intersect the line of sight close to you on its way up (this is called the NEAR zero) and will cross the line of sight again on its way back down (this is called the FAR zero).

From the muzzle to the Near Zero the bullet is below the line of sight. From the Near Zero to the Far Zero the round is OVER the line of sight. From the Far Zero on the bullet is below the line of sight again. Got it?

For the 5.56 round if you want a FAR zero of 400M, the NEAR zero is around 25 yards. So if you zero at 25 yards your bullet will pretty much be zeroed at 400 (its not exact - but close enough for govt work). That is why we then say go to 100 to further refine the calibration.

Generally for AR height sights (2.6" over the bore) you have the following near/far zeros:
25M/375M (or 25 yards/ 400M)
36y/300M
50y/200M
100y/100y What? why is the far zero the same as the near zero? Because the round is already running out of steam being shot at such a shallow angle. So if you zero at 100 yards - that is pretty much the max height the round will reach.

Shoot Straight,
Forest
P.S. where in CT are you from? I grew up in Durham.
Link Posted: 11/13/2002 2:40:22 PM EDT
This picture from the website will help alot:
groups.msn.com/TheMarylandAR15ShootersSite/webpagephotosii.msnw?action=ShowPhoto&PhotoID=502 (click on the link).

The lines in blue are 25 Yard zeros. You can see how they start out below the line of sight [LOS] (which is 0) cross the LOS at 25 yards and keep going higher then cross down over 400 yards away (in the 400 meter range).

If you want to see this picture better you can download the excel spreadsheet with it from our Documents section (Test Results folder, the file is Ammo Track.XLS)
Link Posted: 11/13/2002 2:56:34 PM EDT
FOREST: I cant thank you enuf for your excellent explaination on my question!

So, the bullet is "rising" to the line-of-site for the first 25 yards on my 20" HBar, the goes above the L.O.S. from say 26 yards to 399 yards (rough #'s), then crosses the L.O.S. at 400 yards, and then drop below L.O.S. outside of 400 yards.....do I have it?

400 yards equals 25 yards....I see that now!

I will head back to the range this weekend, set the Leapers to "4", and re-zero my elevation at this new BDC setting.

This explains why I was missing all the close-up shots on all the chipmunks and crows last weekend. They are in trouble again once I am re-calibrated.

I live in Ellington, which is northeast part of CT, lots of woods and farmland.
Link Posted: 11/13/2002 3:04:01 PM EDT
Marcus,
Your quite welcome and yes you got it right (except its 400 meters - meaning almost 440 yards).

Shooting small targets with a scope mounted on top of the carry handle is tough. That high height-over-bore means the bullet will generally be a few inches off the line of sight.

Using the irons - follow LTC Santose's Improved Battlesight Zero. Then zap the critters with using the small aperture and the sight bottomed out (the 50/200 zero) the round will stay within 2" of the LOS from 8M to 220M.

BTW if you're scope is mounted on a flattop (and not the carryhandle) then again I'd recommend using the 200M setting since it should be sitting fairly close to the standard 2.6" HOB (height-over-bore).

Ellington? I got a brother-in-law that lives in that area - nice part of the state.
Link Posted: 11/19/2002 6:36:32 AM EDT
BTW, here's the link for the Improved Battlesight Zero
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