The Effects of Bullet Weight, Velocity and Trajectory on Point of Impact
By Chuck Hawks and Jim Fleck
As handgun shooters, we have often wondered what effect changing bullet velocity and/or bullet weight will have on the bullet's point of impact at the gun's zero distance (usually around 25 yards). This is particularly important to know when dealing with a handgun that has fixed sights.
Iron sights on most handguns sit just above the top of the barrel and therefore the line of sight is a short, but variable (depending on the specific model of handgun), distance above the axis of the bore. The sight line very slightly converges with the line of the bore, crossing at some distance down range. In this article we will discuss changing the point of impact in relation to the bore axis, or line of bore; we find that easiest to visualize. Just understand that when the bullet's point of impact changes in relationship to the axis of the bore, it also changes in relation to the line of sight.
Sadly, after decades of experience, we have found that the result of changing bullet weight and velocity is often unpredictable. However, we have concluded the following:
The guiding principles behind these generalities are:
What are the practical implications of this? Here are some generalizations:
Bullet weight is a major component of recoil, so these two statements make sense. Changing bullet weight while maintaining the same velocity is probably the most consistent way to raise or lower the point of impact, particularly if the powder charge can be kept the same, or at least very similar. (The amount of powder used, while usually not as important as bullet weight, contributes to the total ejecta upon which recoil is based.)
Reloaders can often take advantage of these principles, allowing them to adjust where bullets hit from their fixed sight guns. Unfortunately, shooters relying on factory loads often cannot, since the ammo manufacturers usually load lighter bullets to higher velocity in a given cartridge. This is because commercial ammunition is loaded within specified pressure limits and most factory loads are close to maximum loads. A lighter bullet is typically factory loaded with more powder, and therefore to a higher muzzle velocity, than a heavier bullet loaded to the same maximum average pressure. Match ammunition and cowboy action loads are exceptions to this general rule.
A lighter bullet tends to decrease recoil, thus lowering the point of impact, but higher velocity flattens trajectory, decreasing bullet drop and raising the point of impact. Switching to a heavier bullet increases recoil, causing more muzzle flip and raising the point of impact, but the heavier bullet usually exits the muzzle at a lower velocity, increasing bullet drop, which lowers the point of impact in relation to the bore axis. The effects of bullet weight and velocity on point of impact are therefore opposed, making the net result of switching factory loads hard to predict.
About all a shooter relying on factory loads can do is to be aware of these general principles and try to act accordingly, within the limits of factory load availability. Factory loads are not all created equal; some are "hotter" than others. Trial and error with a variety of factory loads becomes the only recourse, which can be expensive and frustrating.
It should go without saying, but we will say it anyway: another crucial factor is the firmness of the shooter's grip. A firm grip with more muscle tension reduces muzzle flip and therefore the point of bullet impact. A light grip with less muscle tension allows the pistol to kick higher, throwing the bullet higher. Holding a handgun exactly the same way with the same amount of pressure from shot to shot is absolutely critical to maintaining a consistent point of impact with any load. This should be obvious, but there is a natural tendency to "clamp down" on a pistol when shooting heavy loads. For example, a shooter with a .38 Special/.357 Magnum revolver may unconsciously grip the revolver tighter when shooting full power .357 Magnum loads than when shooting light .38 Special practice loads. Any attempt to change the point of bullet impact by changing loads requires using exactly the same grip on the gun with all loads.
Copyright 2013 by Chuck Hawks. All rights reserved.