Want to start loading better ammo? You’re going to need a chronograph. In this article I want to look specifically at chronographs and their role in the reloading process. But first, let’s talk a bit about “Power Factor.”
Power Factor
Put simply, power factor is a measure of the recoil force exerted by a round of ammunition when it is fired. For recoil operated semi-automatic firearms, this is often a measure of how well the ammunition will feed during the load process following a round being fired. If the power factor is too low, insufficient energy will be transferred to the recoil spring and the load process may fail. I find, for my Beretta 92, that any ammunition below a power factor of 120,000 will not feed reliably.
For revolvers and bolt operated rifles, the power factor is of less importance; however, if the power factor is too low, it will generally mean you will have a low muzzle velocity, which will affect the ballistics of your ammunition and hence change the accuracy of your shots. Consistent muzzle velocity/power factor is one component of accurate ammunition.
Power factor is calculated by multiplying the weight of the projectile (in grains) by the muzzle velocity (in feet/second).
Power Factor = Projectile Weight x Muzzle Velocity
E.g., a 145 grain projectile with a muzzle velocity of 1000 feet/second has a power factor of 145,000. The 1000s are often dropped and the power factor referred to as simply 145. The extra multiplier of 1000 is assumed.
For competition purposes, many matches specify a minimum power factor, which may be anything from 60,000 to 180,000 grain feet/second (60-180), depending on the type and class of match.
Chronographs
When determining the load characteristics for your reloaded ammunition, reloading tables are a must. These tables will give you the correct powder charge given a number of parameters including the caliber, powder type and projectile type/weight. These tables generally give you a start powder charge and a maximum powder charge, which ensures that you won’t exceed the maximum pressure in the case. Unfortunately, it is not possible for most reloaders to measure the case pressure, so we go to a secondary measurement, muzzle velocity. Reloading tables generally quote the expected muzzle velocity for a specific load configuration. By checking the muzzle velocity, we can be reasonably confident that the load is in accordance with the reloading tables and hence the case pressure should be within limits.
To measure the muzzle velocity, we use a device called a chronograph (or “chrono,” for short), which, as the name implies, actually measures time, not velocity (or more correctly, speed). A typical chrono is shown in Figure 1. These devices generally cost around $200.
Chronos work by measuring the time taken for a round of ammunition to travel a specified distance and translating this to a velocity. Most modern chronogrpahs also have the ability to analyse a number of shots and calculate average values as well as statistical data, which can be downloaded to a computer for recording and later comparison. They may also be able to take an input of the projectile’s weight and directly display power factor.
Image Courtesy of www.MidwayUSA.com
In operation, a round or series of rounds are fired through the area beneath the two screens of the chrono, generally from a distance of 6 – 10 feet back from the first screen (Figure 2). This distance is necessary to ensure that the projectile has established a steady state velocity and that interference from muzzle flash etc will not adversely affect the chrono. Sunlight (or an artificial light source) coming from the top of the screens casts a shadow on a sensor below each of the screens. As the projectile passes the first (start) screen the timing starts, and as the projectile passes the second (stop) screen the timing stops. The screens are a known distance (around 2 feet) apart depending on the make of chronograph. The time difference between the start and stop signals is calculated and translated to a velocity.
Chronos are a relatively expensive item, especially if you only need them occasionally to confirm your reloads are correct. However, for competition shooters, or those who enjoy tinkering with getting their loads “just right,” they are an invaluable tool.
If you don’t want to invest in a chronograph, or can’t borrow one when necessary, it may be best to stick to the lowest recommended powder charge, or the lowest charge that gives you reliable operation of your semi-auto. NEVER exceed the maximum recommended powder charge for your specific load configuration.
We’ll tackle the load development question further in later articles.
Next Articles
Having now covered the basic reloading tools and dies as well as some other cool stuff, it’s almost time to look at the actual reloading presses that utilize these dies/tools. Firstly however, I’d like to look at one of the parameters you may consider when determining your reloading choices, the reloading rate. Understanding the reloading rate of a specific approach/equipment configuration will help you make an informed choice when it comes time to open your wallet .
