Knowing the actual velocity of your specific ammunition out of your actual firearm can be very useful. With that data, you can tell:
- How much drop compensation to dial in at a long-range.
- What power factor you have for IPSC.
- Which load develops higher velocity in your particular gun.
For example, 100fps deviation from nominal velocity listed on the ammo box can mean a foot difference in drop at 500 yards. Different brands of .357 Magnum 158-grain ammunition can result in between 1630fps and 2250fps from the same 18-inch carbine.
Counter-intuitively, the lightest 110-grain can actually be slower than the top 158-grain. So, how do you know the true velocity of your ammo? With a chronograph, of course.
What Are Chronographs?
The oldest chronographs date back to the 1890s. Greatly improved variants are available today from several makers. They all measure velocity by registering shadows from bullets across optical sensors, then comparing the timing between the front and the rear sensors.
They work very well outdoors, but can be rather more difficult to use in dimly lit indoor ranges. There’s also a constant danger of hitting the sensor themselves, as the bullet must pass within a foot or so of them.
Types of Chronographs
Two competing concepts have recently appeared. The first is radar, represented by FX Airguns (for air gun pellets and bullets under 1000fps) and LabRadar (for all firearms).
It can be tricky to produce correct alignment and offset of the sensor, especially with tapered barrels or integral bayonets. MagnetoSpeed works well with shotguns and rifles, but will not fit most semi-auto handguns.
What Chronographs Tell You
All of these devices will save and aggregate ballistic data, allowing to determine velocity spread and standard deviation (SD) from mean to better judge the consistency and the performance of various loads.
For example, testing Seismic 185-grain plated hollow point, I got an SD of seven, a terrific result. Old Chinese 7.62×39 surplus has an SD exceeding 100, a clear indication of either poor quality control or improper storage—both good reasons to discontinue shooting it.
With a chronograph, we can also find out if a particular rimfire load breaks the sound barrier or not, thus predicting its noise level in a sound-suppressed firearm.
Do you use a chronograph? Let us know your thoughts in the comments below.