While this isn’t really a “tactic” per se, it is a fundamental aspect of weaponcraft that is vital to understand when you are using a long range weapon. The thing I am speaking about is what is referred to as a weapons “zero”.
To understand what a “zero” is, you have to understand the fundamental physics concept of trajectory. A bullet does not travel in a straight line from the barrel to the target. It travels in an arc. Depending on the weight of the projectile, the velocity the bullet is traveling at and various other influences; a bullet rises to a maximum height and then begins to descend under the influences of gravity and the projectiles gradual lessening in velocity as it travels through the resistance of the atmosphere.
In practical terms what this means is that you have to adjust your weapons sight so that it intersects this arc at a known point. Where your line of sight and the path of the bullet intersect is called that weapons “ZERO”. The bullet is not above your line of sight (+) or below your line of sight(-)…it is at 0 deviation. This way you know that your bullet will strike where you are aiming if the target is at that “zero” distance.
A weapon is “ZEROED” at two points along the bullets path; where the bullet rises to your line of sight and where it falls once again to your line of sight “downrange”. The standard military “ZERO” is at 25 meters and (depending on bullet type) 250 meters.
As a rule of thumb you need to remember that if the target is closer than your “near zero” then your bullet is going to strike low. Between your “near” and “far” zero the bullet is going to strike high. And beyond your “far zero” the bullet is going to start dropping below of your line of sight.
Tinkering with the distances you can zero a weapon at is a topic of much debate. There are varying schools of thought on what the best distance to zero your weapon is. It’s my humble opinion that the “best zero” is the one YOU can remember and utilize most efficiently. I have been schooled and am most experienced with the 25M military zero so that’s what I stick with. Your personal mileage my vary.