A negative lead is typically used when engaging a stagnant target while the shooter is moving. A negative lead can also be used on a moving target.
Negative leads are typically seen in high-wind velocities, when the target speed is relatively slow. Let’s use the following as an example:
- A 175-grain .308 HPBT bullet
- Bullet velocity of 2650 FPS
- A wind velocity of 10 MPH at a 270° angle
- A distance to the target of 400 yards
Negative Lead Calculation
With the information provided above, in order to hit a stagnant target at this distance, we know that we will need a wind hold of approximately .91 MILs in the direction of the wind. If the target is moving at a speed of 4 MPH with no wind, the lead will be around 2.4 MILs. If the 4 MPH target is moving in the direction of a 10 MPH wind (left to right), we will need a lead of 1.7 MILs as opposed to the -3.5 MILs if the target was moving right to left. The reason for the drastic MIL hold is due to the fact that we are not only fighting the wind velocity, but the lead as well (when the target is moving into the direction of the wind).
With a wind velocity of 20 MPH moving the same direction of the target at 270°, we will need a lead of only 0.8 MILs. The lead value decreases because we are using the velocity of the wind to help ‘push the bullet’ into the moving target. The lead value drastically decreases when the wind’s velocity exceeds 30 MPH. So for a target speed of 4 MPH, moving left to right, with a wind velocity of 30 MPH at a 270° angle in relation to the shooter, we can expect a hold of around -0.1 MILs. For a target that is moving at a slow walking pace with the direction of the wind, blowing at 2 MPH, the lead hold would be -1.4 MILs.
Although the wind speeds may be extreme to some, note that the target speed is 4 MPH—a relatively fast walk—and that the distance to the target is a mere 400 yards. For the extreme precision shooter, we may encounter moving targets at ranges of 600-700 yards.
Using a slow walking pace of 2.5 MPH, moving with the direction of a left-to-right 25 MPH wind, at a range of 700 yards, our lead would equate to -2.7 MILs. Decreasing the wind velocity to a mere 15 MPH, we will need a lead of -0.9 MILs.
As a rule of thumb, with most .308 loads, the following formula will work for targets at a distance of 500 yards or less, moving at right angles to the line of fire:
Lead in MILs = target speed (MPH) * 0.6
Typical human/target speeds
- Slow patrol = 0.8 mph
- Fast patrol = 1.3 mph
- Slow walk = 2.5 mph
- Fast walk = 3.7 mph
- Jogging = 6 mph
- Fast run = 11 mph
Considering human/target speeds on an average walk or stroll, the negative leads may not be as great. But as a sniper/precision shooter, we know that shots in wind, on a moving target, are often a miss on the first shot, and after the target knows that it is being engaged, it will begin to run or jog.
Looking at several data charts and mathematical target lead tables of several bullets, I have found that a ‘negative lead’ will greatly apply at wind velocities exceeding 30 MPH, when the target is moving at a slow to fast walk, and also with a wind velocity of 10 MPH, when the target speeds are at a slow patrol.
The shooter should also be aware of time considerations when engaging moving targets. The bullet does not leave the barrel as soon as the sear releases; rather, there is a delay between the release of the sear and the bullet exiting the barrel. During this time, any movement can move the firearm off target, and so this time should be minimized, especially for firearms that will be fired from an unsupported standing position. This delay can be broken down into three sections, the lock time, bullet dwell time, and shooter reaction time.
These time considerations are extremely important to a shooter engaging moving targets using formulas or ballistic calculators. Formulas and ballistic calculators do not account for time considerations, but instead are only true/correct when the bullet clears the tip of the muzzle.
Lock time is the time between the release of the sear and the ignition of the cartridge. A lengthy lock time gives time for the shooter to drift off target, and so it is advantageous to minimize the lock time, and thus reduce the window for error. Reductions in lock time are generally performed by lightening parts that move as part of the firing operation, such as the hammer and firing pin or striker, and using a more powerful spring. Further reductions in lock time, to near-zero levels, can be achieved with electrical primers.
Dwell time (in regards to the bullet)
The bullet dwell time is the time between cartridge ignition and the time the bullet leaves the barrel. Like lock time, dwell time is a window for error, and can be minimized with a faster bullet or a shorter barrel. In some cases, a shorter barrel is desired to reduce dwell time, but without losing the sight radius of a longer barrel. In this case, a sight extension tube, or bloop tube, can be used. This is a tube that fits on the muzzle end of the barrel, providing support for the front sight, but that is bored much larger than bore diameter. This provides the sight plane of a long barrel with less weight and dwell time.
Shooter reaction time
The time that it takes the shooter to visually see the target intersect the desired hold in the reticle, and the time it takes the shooter to fire, is shooter time. Some shooters may perceive things differently or may have faster reaction times than others. This variance in reaction time can be seen greatly in the elder and younger shooters. Typically, the younger shooter will have less shooter reaction time than the elder shooter.