According to Brownells the front and rear sights should be the same distance above the bore center line:

quote:

Sight Height Calculator

Proper sight height is not difficult. Just remember that the goal is to make the front and rear sights the same height above the center of the barrel. And, when you make your measurements, you MUST use a caliper or micrometer. You must have a decimal answer (for example, .875) for the calculator to work. A few thousandths can have a substantial effect and this is no place for a tape measure or micrometer eyeball!

Obviously this would have the line of sight parallel with the bore, but it would have the bullet hitting low at all distances. So, if the bullet is to hit point-of-aim at a given distance the rear sight must be higher above the bore than the front sight.

So my question: if I want to make a "standard big bore" (a rifle that pushes a round nose bullet with a SD of .300 at 2400fps) hit dead on at 100 yards, how much further above the bore should the rear sight be compared to the front sight(given an 18" sight radius)?

As close as I can figure most of the rifles I have that fit this description(2400-2700fps, hitting dead on at 100Y) have a rear sight that is about .050" taller that the front sight(measured from bore center-line)according to my measurements.

I guess I just want to know if this sounds correct to you guys. This works out to about 10 moa. I guess I am surprised that 10 moa of elevation is required to hit dead on at 100 yards.

Am I measuring something incorrectly? Is 10 moa of elevation really required to hit POA at 100?

Is their a calculator that would take the ballistics or a certain load into account to give the required sight elevation for predetermined distance?

What you do is set the rear adjustable blade at it's midpoint of adjustment to get your above bore line measure. Then that gives you flexibility to choose a proper front sight height. If not adjustable, you either have to select a front ramp and bead combo or a different rear leaf height.

You would need to use a ballistic calculator to determine how much drop you are getting at 100 yds with your load. Add that to the above BL measure and you have how much you need to compensate.

Example: Lets say your load drops 1 inch at 100yds and your front sight is 1 inch above the bore line. That means your initial setup needs to move the bullet 2 inches up at 100 yds to hit dead center(if your aim point is the tip of the front sight). Your rear sight needs to be higher than the front to achieve that. Since your sights are exactly 18 inches apart and your range is 100 yds, or 3,600 inches, you can calculate the amount required. Sight spacing/range ratio is 18/3,600 or .005. Since that is a direct proportion, it means every .005 change in sight height results in a 1 inch change in bullet impact at 100 yds. So if you needed to move the bullet 2 inches it would take 2 x .005 or .010 inch increase in rear sight height to move it.

If you are filing sight leaves to adjust, start out a little high and do the final adjustment at the range with the load you definitely will be using. In most cases, you can start out with a rear sight .020-.030 higher than the front and just go to the range with a file and fine tune.

EDIT: A ballistic table I consulted showed a .416 Rigby 400 gr RN, BC .316, at a MV of 2,400 fps, drops about 3 inches from line of sight at 100 yds. Using that, you would have that plus your height above BL which may be 1-1 1/2 inches. So using our calculation above, you would need to move the bullet around 4 inches or so. That would be close to .020 in sight change.

Bobs answer address the math and the relationship between line of sight and bore center line.
And he did mention a ballistics calculator to determine drop.

There are a few calculators on the web but you need to remember you are trying to find the drop below line of sight. Some calculator just give you the general drop. some don't even take into consideration the sight height above the barrel. which is all critical.

And then there is the ballistics a .220 swift will have less drop the a .308. and a lighter bullet will drop less as well too. so there are a bunch of thing to consider.

Here's how I do it: Take it to the range, fire a shot, file the sight a little, shoot again and continue until it hits where I want it to, but then I was never good at math. Of course, this does waste ammunition; the last one I did took seven rounds.

They quickest way for me to lose interest is to introduce math into the equation. The "bullet and file" method works for me. Life's too short to be bogged down with science or math.

That's good way to do it and nothing wrong with avoiding math. I do it all the time.

But start on the high side of things. Because it's easier to file off metal then it is to file it back on. And rubbing on it will not make it grow.

I have tried the doing the math for big bores and have not been very sucessful. What I do for bore bores is to put the front sight on with the lowest bead available. Then I put the rear sight on with some sort of clear adhesive (like silicone sealant) that will allow the sight to stay on the barrel for sight regulation but is easily removable. The rear sight will be permanently attached afterwards when the regulation process is completed.

I then remove the rear sight blade insert and glue a piece of card stock in the rear sight blade slot, making sure it has sufficient height.

Off to the range at 35 yards with my ammo and a pair of scissors. Fire a few shots, trim the rear sight insert until it is zeroed and I am done. Now I know the exact height necessary for the rear sight , with the ammo I will be using. If need be I can re-position the rear sight, or put in a higher front sight blade.

Hope this helps.
Easy to do without a lot of fuss.
Math free