In an effort to explain this subject in the shortest way, I will have to simplify. I will use the 995 as the test bed. Temperature 65ºF, barrel length 17", no wind, altitude sea level and gun is held level. The target is at the same height as the sights and not the barrel.
Line A is the the barrel.
Line B is the top rail, level with the barrel.
Line C is the Line Of Sight (LOS).
Line D is the height of the rear sight aperture above the barrel center.
Line F is the height of the top of the front sight post.
Line E is the height of the rear sight aperture above the top of the top rail.
Let's set up for our first shot. Set the rear sight to the middle of its range using the lines at the back of the sight. This distance (E) is about 22mm above the rail. I measured the distance of the top of the rail (B) to the barrel bore (A) to be about 35mm. So the total of A to C is 57mm. Set the top of the front sight post to be about 57mm above line A. This distance is represented by the line F.
Use a target set at 25 yards. Make the target level with line C.
Fire! Where is the bullet hole? No wind, the bullet goes straight. At 25 yards the bullet would have dropped by 0.8 inches. The distance from the line of the sight (C) to the bore (A) is 57mm or about 2.24" and plus the 0.8" drop, that means our bullet hole should be 3.04 or say 3" below the bulls eye.
It's obvious that the carbine must be pointed at an up angle to hit the bulls eye at 25 yards. So we tilt the gun up and now our sights are not pointing at the bulls eye. So, we adjust our sights, we can raise the rear sight or lower the front sight to get on target. I would lower the front sight a little, 1 or 2mm, and shoot again. The bullet hole is now a little closer to the target and I would start adjusting the rear sight next. So now it's, take a shot, adjust the sights until you are hitting the target. In an ideal setup, your gun would be in a vise. With the gun level and the first bullet hole 3 inches low, you loosen the front sight, look thru the rear sight and lower the front sight until it was pointing at the bullet hole. Then you would raise the barrel of the gun until the sights were on target and take your shot.
Where will the bullet hit say at 25, 50, 75 and 100 yards?
In reference to the LOS, the barrel is pointed up, but the bullet is being pulled down by gravity and so what happens. The muzzle velocity, weight, ballistic coefficient, height of the sights above the barrel and the pull of gravity effect what happens. Gravity pulls the bullet down and as the bullet slows down due to air pressure the downward curve increases.
Take these things into consideration and look at what happens to our bullet at 25 yard intervals. I chose a 115gr FMJ bullet with a muzzle velocity of 1300 ft/sec, a ballistic coefficient of 0.130 and a sight height of 2.25". (ToF is time of flight)
The bullet leaves the barrel 2.25" below the LOS, at 25 yards it has risen to hit the target dead on, at 50 yards it is 0.76" above the LOS, at 75 yards it has dropped to 0.18" below the LOS and at 100 yards it is 3" below the LOS.
Let's look what happens if I sight in at 50 yards. With a 50 yard sight-in, the bullet rises at a less steap angle and at 25 yards it is 0.38" below our LOS. At 50 yards it is dead-on and at 75 yards it has dropped to 1.31" below LOS and at 100 yards is 4.52" below the line of sight.
The impact point is ok out to 50+ yards, but 1.31 inches low at 75 yards and 4.52 inches low at 100 yards. A flatter shooting round can be sighted in at a further distance and still give good results at a 100 yards, but not the round I choose to use.
I only recommend sighting in at 25 yards as a starting point. If you change the weight of the bullet, its muzzle velocity or its shape then the shape of the trajectory curve will change and you will have to compensate for it.
You have choices. Sight in with the ammo you plan to use the most. If you want to try other ammo, you do not have to change your sights, just concentrate on grouping.
If you change your ammo, then adjust the sights for its characteristics.
This information is just a jumping-off place for you to make all the adjustments to fit your particular situation.
If you would like trajectory tables go to:
Good information at Harnady: