Which type of cartridge's headspace is measured at the datum line?

The headspace of a cartridge is a critical measurement that ensures proper chambering and firing of the round in a firearm. For bottleneck centerfire rifle cartridges, headspace is specifically measured at the datum line. The datum line is a defined point on the cartridge case that allows for consistent measurements across different cartridge specifications.

In a bottleneck cartridge, the shape of the case is designed to taper toward the bullet, which influences how the cartridge fits in the chamber. The datum line is located at a point where the case shoulder meets the body of the case, making it crucial for alignment when the bolt closes on the cartridge. This precise measurement helps to prevent issues like excessive headspace or poor firing conditions that can lead to malfunctions.

Understanding this concept is vital for anyone involved in firearms inspection and troubleshooting, as proper headspace measurements contribute to the safe and effective operation of firearms.