Until you understand, it may be confusing to test your handloads on a chronograph and find the velocity is not what you expected based upon the reload data. There are a number of reasons for this. We will attempt to clarify this mystery with the following:
- Hodgdon reloading data is fired from Industry-Standard Test Barrels
The test barrel is a precision piece of ballistic equipment. To yield the greatest consistency in data, the test barrel has very precise chamber dimensions and bore dimensions. Your firearm, although a finely crafted machine, is built on a production line where tolerances and tooling wear occur. Not only do different gun types vary in dimensions and tolerances, but even two of the exact same gun model can show dramatic differences in velocity produced.
- Test barrels generally do not have cylinder gaps
If you are a revolver shooter, the cylinder gap will cause your velocity measurement to be lower than the reload data due to gas loss in the gap.
- Test barrels have a fixed length
In centerfire rifles, the test barrels are all 24-inches long. Your 26-inch varmint rifle will shoot higher velocity, and your 18-inch carbine AR-15 will show lower velocity.
- Velocity measuring screens are set at an exact distance apart (10 feet) with the center of the screens exactly 15 feet from the muzzle
Few handload type chronographs have the screens separated apart more than a couple of feet. Generally, at the range, chronograph distance from the muzzle of the gun is not fixed or exact.
- Muzzle flash and blast can impact the recorded velocity
Ballistic labs generally protect the screens from muzzle flash and blast.
This does not mean your chronograph setup for measuring handloads is not useful. Just be aware of these factors and establish what level your particular setup is at. One way to help establish where your test setup is shooting is to fire some factory loads and compare your results to the factory published numbers.