How much to adjust the scope to shoot 500 yards?

[Dave King]

Most long range shooters don't use 'hold-overs' to shoot. We typically don't sight our guns for 2 inches high and 100 yards and then hold high to make the shot. We most frequently zero the scope at 100 yards and then create a ballistics chart to use when shooting.

To begin making a ballistics chart for your rifle you'll either need to consult a ballistics program or table. You'll need to know the speed of the bullet from YOUR gun. You'll need to know some particulars about the bullet, the Ballistic Coefficient (BC).

Begin by zeroing the rifle at 100 yards, this should be Point Of Aim (POA) to Point Of Impact (POI). Consult the ballistics chart for the 200 yard drop info. If the bullet drops 4 inches at 200 yards with a 100 yard zero you need to move the scope's crosshairs up 2 MOA to get the bullet's trajectory to cross the 200 yard line at the center of the target.
This is determined in this manner, the scope is 'dead-on' at 100 yards and the bullet drops 4 inches at 200 yards. We need
to adjust the scope so the bullet has an additional 4 inches of ballistic arc at 200 yards. Because the sight mechanism on a rifle is an angular system we need to convert to an angular measurement. We use Minute Of Angle (MOA) as the system of measurement for sight adjustments. To convert 4 inches drop at 200 yards to MOA we must divide the inches of drop by the distance in hundreds of yards. It looks like this:

4 inches / 2 hundred yards = 2 MOA.

We must come up on the scope adjustment by 2 MOA ( 1 MOA is actually 1.047 inches but for this example we'll use 1.0)
We now have the come-up data for the 200 yard shot and we can proceed to the 300 yard data.

We've covered this once and it's the same thing for ALL distances. Find the bullet drop for 300 yards with a 100 yard zero... we'll call it 15 inches. Divide the inches of drop by the hundreds of yards;

15 inches / 3 hundred yards = 5 MOA of drop.

The come-up data for 300 yards is 5 MOA over the 100 yard data. (But it's only 3 MOA over the 200 yard data.)

Now we'll do the 400 yard data... call the total drop 34 inches with a 100 yard zero.

34 inches / 4 hundred yards = 8.5 MOA over the 100 yards data.

Last one for this example. 500 yards data is 60 inches of drop with a 100 yard zero.

60 inches / 5 hundred yards = 12 MOA over the 100 yard data.

So, to adjust our theoretical rifle to make a Point Of Aim to Point Of Impact hit at 500 yards we adjust the scope UP by 12 MOA from our 100 yard zero. 12 MOA would equal 48 'clicks' on a .25 MOA (1/4 inch) adjustment scope. Hold straight on the target and press the trigger for a 500 yard hit.

[Ian M]

Dave,
Since I do not use any computer trajectory programs I place targets at each distance and try to "catch" the shots on paper to determine come-ups. Fortunately doing this with the .308 and .300 Win. mag is very simple since there is so much drop data available. The .338 or .257 Weatherby might be an entirely different matter.

Do you put complete faith in computer generated drop and wind figures? I wonder about variables - how close would drop figures be for your Remington vs my Winchester, your MK4 mounts vs my Badgers etc.

Not trying to disparage your tables, just interested in your opinion as to how accurate they really are.

[Dave King]

Ian

I use the drop tables generated by the computer programs as a starting point. They're generally very good but too many variables to be completely relied upon.
Once I have a set of computer generated I check the values against actual field data and use the known good field data as the rifle chart.

Sorry I didn't clarify that point very well.

I don't believe the mounting system effects the drop tables given that the mounts are the same height. Some mounts give a greater sight axis to bore axis distance, usually folks pick 1.5 inches for sight/scope height but for some of my high rings and the bigger 50mm objective scopes it may be as much as 1.75 or more.

I haven't seen any difference based on the rifle manufacturer if the actual bullet (chronographed) velocity is the same. But I'd guess that the way a bullet is engraved during it's time in the various barrels would cause the final effective Ballistic Coefficient to be different and that could cause some degree of variation at long range.

As for 'guessing' as to what a particular rifle's trajectory might be it no other data is available... I know the come-up data for the 308 Win and the 173 military match bullets. I call this data my standard set... for the faster magnum cartridges and the little fast varmint rounds I use a figure that's 2/3s the 308 come-up data, this usually gets me very near the proper value so I can get known good data from hits at various distances.

[Len Backus]

Dave

I think I read somewhere that some of the variation in actual observed BC of bullets may be due to pitch or yaw in flight. This observed BC can actually vary with the same lot of bullets if fired from 2 different barrels or from two different loads with different powder/seating depths/etc combinations.

[Tim Behle]

Dave,

I just wanted to thank you for all of the help you and this site have been. Down towards the beginning of this forum, you have quite a few posts that have been an immense help to me.

This post in particular turned on new lights for me. I've been using a ballistics program for several years. While I always looked at the drop charts it gives me. I never thought them good for anything but to help me get on paper, and only out to 300-400 yards.

No one ever explained to me I need to divide the inches by the yardage to get distant MOA's. Last night, I took a drop chart I had made myself, and compared it to my ballistics program. Followed your division and it was with in a couple of clicks, every 50 yards all of the way out the the 600 yards I have practiced.

Keep on writing buddy, I'm memorizing as fast as I can! [img]images/icons/grin.gif[/img]

[bones]

Ian, you said theres a lot of drop Data out there ,wear can I get it for the 300 mag ?

[Ian M]

I use the drop info for the Black Hills Ammo .300 Win. mag with 190 match bullets from their Ballisticards. Here are the basics:
Yds IN. MOA
100 0 0
200 -2.8 1.3
300 -11 3.4
400 -24 5.8
500 -44 8.4
600 -71 11.4
700 -107 14.6
800 -152 18.2
900 -208 22.1
1000 -277 26.5

Your loads might vary a bit but this is good starting info. If you load a different bullet you might want to check out www.ballisticards.com for info on getting custom drop charts. They are made up on heavy laminated cards - they are great. He gives wind deflection, leads, angle shooting hold-offs in the charts for your particular handload as long as you give him some base data.