I recently bought a S&W 15 (16" barrel) and due to the weather, I can't get out to shoot. So I decide to dress it up.
New grip, bipod, sling, and Nikon P223 BDU scope. More stuff to put on later. Now the question: I set it up in the basement and lined the scope up and bore sighted it. I want to zero the scope in at 100 yards. Right now the crosshairs are about 4 inches above the laser dot. Can anyone tell me what the (approx) drop would be at 100 yards using a 5.56 - 55 grain round? Any help would be greatly appreciated.

What is the distance between the center of the bore and the center of the scope (line of sight)?

What is the distance from the muzzle to the wall you're looking at?

Both distances matter, the first a lot more than the second.

Assuming you have a 2.5" distance between the bore axis and the line of sight, and a carbine to wall distance of 10 yards (30 feet) the distance between a bore sighting laser and the crosshairs should be around 2.1".

If the distance between bore axis and line of sight is 1.7", then the distance between the laser dot and the cross hairs should be around 1.3".

BB57
Thanks for the reply.
Here's the info you requested.
The distance between the bore and the center of the scope is about 3 1/4 inches.
The distance from the muzzle to the wall is 19 feet (give or take an inch or two).
Hope this helps.

Using those numbers... multiply by... over x... carry the one.... I have no idea.

Just take it to the range, put the cross hairs on the dot, fire a shot and go from there... so much easier.

That would be a lot easier but it's 10 degrees outside. Just trying to get an idea on where it'll hit.

The ballistics program cranks out 2.9".

That should get you on the paper at 100 yards.

It's absolutely vital however that the rifle be level and the reticle on the scope be leveled to the rifle when setting it up.

3.25" is a long way above the bore so canting errors can be significant.

Leveling an AR-15 is pretty easy if you have a rest, a gun vice, or a bipod to keep it level along with a household level to level it. You just need to find a flat on the rifle to set the level on. The scope can be ball parked leveled across the adjustment turret or cap, but some scopes only come within 3-5 degrees of level that way.

Another, and better, alternative is to sight on a mirror placed far enough away to allow you to focus on the image. Once you've got focus, you rotate the scope in the rings so that the vertical crosshair (or the edge of it, if it is thick) passes through the center of the scope and the center of the bore. That will level the crosshairs, and ensure when they are level while you are shooting that the line of sight is directly above the bore with no canting of the rifle.

BB57
Thanks a bunch. You've been a big help. When I leveled the scope, I hung a weight on a string from the ceiling of my basement and lined up the vertical crosshair with the string. I really like the idea of using a mirror. I think I'll double check the scope with the mirror. Now all I need is warm weather. Old bones don't move to good when their cold.

Another method....

A. Remove lower
B. Remove BCG
C. Put upper on something relatively steady
D. Look through bore, aim at a point at distance
E. Remove hands
F. Look through scope, estimate difference, adjust accordingly

Repeat D-F as necessary.

The idea is to be on paper when you get to the range

Here's a link to a handy ballistics calculator with an assortment of manufacturer data for many calibers, and weights. Under advanced options, you can select zero distance, and scope height over bore. The tool will show you bullet trajectory for the distance and intervals you pick.

http://gundata.org/ballistic-calculator/


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