How to sight black rifle at 20 yds for even at 100?


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bushmaster1313
August 21, 2010, 10:50 PM
How do I sight in at 20 yards for even at 100?

Do I want to be high or low at 20?

How much?

Scope is a fixed 4x Leupold tube on rings.

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bushmaster1313
August 22, 2010, 01:12 AM
I just read that even at 25 yards equals a bit high at 100, even at 200 and a bit low at 300

Zak Smith
August 22, 2010, 02:20 AM
Not even close.

The average sight over bore distance for a properly set up AR-15 is 2.6-2.7".

Here's data for a 100 yard zero and 2.6", an average load from a 16".

_Bullet_ _BC_ _MV_ 0 10 20 30 40 50 60 70 80 90 100 | YARDS
M193 0.243 3070 > -2.60 -2.16 -1.75 -1.39 -1.06 -0.77 -0.53 -0.33 -0.17 -0.06 0.00 | drop (inches)


A 25 yard zero is a bad idea

_Bullet_ _BC_ _MV_ 0 50 100 150 200 250 300 | YARDS
M193 0.243 3070 > -2.60 2.35 6.26 8.96 10.30 10.05 7.99 | drop (inches)

U.S.SFC_RET
August 22, 2010, 07:09 AM
Not even close.

The average sight over bore distance for a properly set up AR-15 is 2.6-2.7".

Here's data for a 100 yard zero and 2.6", an average load from a 16".
Code:

_Bullet_ _BC_ _MV_ 0 10 20 30 40 50 60 70 80 90 100 | YARDS
M193 0.243 3070 > -2.60 -2.16 -1.75 -1.39 -1.06 -0.77 -0.53 -0.33 -0.17 -0.06 0.00 | drop (inches)

A 25 yard zero is a bad idea
Code:

_Bullet_ _BC_ _MV_ 0 50 100 150 200 250 300 | YARDS
M193 0.243 3070 > -2.60 2.35 6.26 8.96 10.30 10.05 7.99 | drop (inches)


If you are shooting ground hogs at 250 then a good zero at 25 ought to do you.
I would trust a 25 yard zero any day of the week when it comes to any fair distance shooting. Shoot a couple of tads low at shorter distances while distance shots are easier to hit
It depends on how you were taught. I was taught a 25 yd zero in the military and never had a problem shooting the high scores.
There is a mindset that a 100 yd zero is a must. The wanting to drill through a coin at that distance because you are trusting a 100 yd zero is fixated with a lot of shooters.

bushmaster1313
August 22, 2010, 08:30 AM
Here's the issue

Just reconfigured the scope on my Bushmaster and I can get to my local 20 yard range but not my 100 yard range.

So the question is, how should I zero at the close distance, which is all I can do for now?

Picture before taking the rail riser and BUIS off and switching to a thinner pad:
http://i700.photobucket.com/albums/ww6/bushmaster1313/CIMG3561.jpg

(BTW gun and magazine were checked twice for empty)

Kwanger
August 22, 2010, 08:45 AM
As per Zak's chart, I'd zero so the point of impact is about 2.6 inches low at that short distance, then look to get to a longer range to confirm and fine tune.

bhk
August 22, 2010, 09:29 AM
Your scope is really mounted high, probably negating Zak's chart to a degree. You would have to sight in even lower than the -2.16 inches at 20 yards to achieve an approximate 100 yard zero with the configuration you have.

The old 25 yard zero for a hundred yard dead center hit (that many of us 'older' folks go by) became the norm for traditional-style rifles with very low mounted scopes (center of scope 1.5 inches above the bore) .

docsleepy
August 22, 2010, 11:05 AM
measure the height of your scope above the centerline of the barrel

Go to the jbm ballistics calculator (google it)

type inthe height of your scope, the type of bullet, grains, speed, whatnot.

request a zero at 100 yards, request printout every 25 yards to whatever your want (at least 100)

Hit "calculate"

Read the output. As a check, it should say "0" inches for the 100 yard line.
Now read the 25 yard line, where it will tell you how far above or below the 25 yard shot should go.

Go shoot a round at a target at 25 yards, measure the vertical error, adjust our scope until the error is exactly what the jbm calculator says it should have been in order to be zeroed at 100. That should be pretty close.

Zak Smith
August 22, 2010, 12:38 PM
At 250 yards you'd have to aim 10" low to hit ground hogs with a 25 yard zero. Not a great idea.

If you want to measure the scope center over bore center distance I'd be happy to compute a new table.

I am not advocating for a 100 yard zero in this thread. I posted the 100 yard zero chart because the OP asked for it. When 25 came up, the data was to refute that it'll be a "little high at 100" -- it's a lot high at 100 and higher further out.

Per the other thread we just had, a 50-yard zero is a good choice for a standard AR-15 to maximum a +/- 2" point blank distance, if the sight does not have a means of compensating for drop. For scopes with a BDC reticle or external target knobs (ie a real long range scope) a 100 yard primary zero it a better idea.

Flfiremedic
August 22, 2010, 01:08 PM
Zak can you email me that calculator?

Zak Smith
August 22, 2010, 01:15 PM
Sorry, it is one I wrote and not for public consumption. There are various free ones online that you can use, though.

Palehorseman
August 22, 2010, 01:18 PM
I dunno, but remember even over 50 years ago when in basic training, we initially zeroed the M-I on the 1,000 inch range. This works out to about 28 yards, saved ammo and put us close enough on target for final zero at 100 yards.

ThePunisher'sArmory
August 22, 2010, 01:21 PM
As told to me from Larry Vickers while watching the show Tactical Impact. Your rifle's POI should be 2" low from POA at 25yrds. This should then be checked at 100yrds and should be dead zero at that distance. Edit: This only applys to Iron Sights.

bhk
August 22, 2010, 01:38 PM
The controlling factor in all of this is the distance of the line of sight above the bore. No charts can be made without that information (measured ACCURATELY). The chart that Zach used is for the standard sight height for iron-sighted ARs and for scope mounts that approximate that same height. Many AR scope mounts put the scope slightly lower or higher than standard. If you mount your scope lower or higher (as with the OPs rifle), you need a new chart.

Even with a properly calibrated chart, you are getting only an approximation. I rarely find them to be dead on due to a variety of factors such as bullet weight, bullet BC, actual velocity vs. claimed velocity of load, etc. Close range sight-ins always need to be verified at longer range, and you will almost always have to make tweaks. I guess how much tweaking you need to do depends on your target. If you are shooting at B-27 targets, good enough may be good enough. If you are trying to hit a 1 in. X-ring at 100 or 200 yards, minor adjustments are critical, of course.

basicblur
August 22, 2010, 02:15 PM
You can download free Remington Shoot software here (http://www.remington.com/pages/news-and-resources/downloads/remington-shoot-software.aspx).
Like many in here, I was fed conventional wisdom on zeroing scopes, but it was usually for rifles with scope heights closer to bore center than the AR.
After plugging in the scope height of your choice, the trajectory graphs are real eye openers-should help you decide how you want to zero.

Point Blank (http://accurateshooter.wordpress.com/2008/02/26/free-point-blank-ballistics-software/) software is also free-I have it, but seem to find myself using Shoot more often.

bushmaster1313
August 23, 2010, 12:08 AM
Well, the gun in my post was zeroed in today at 21 yards and about 2.5 inches below target.

Looking forward to taking it to the outdoor range at 100.

BTW, I had much more trouble holding the rifle steady for a good off hand shot than I did with my pistol and its iron sights.

oldbanjo
August 23, 2010, 12:25 AM
In 1969 I purchased a new Browning 3006 auto and a B & L scope the book on one of them said to sight in at 27 yds and you would be shooting in the killing area of a deer from 0 to 350 yds. I've sighted a lot of guns in for other people and I've always used 27 yds. No one has ever complained.

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