Flash Hole Diameter

  • QUESTION: How important is the diameter of the flash hole??? — Boxer primers have only one flash hole [also known as “vent hole”] and Berdan primers have two flash holes. A flash hole can be punched or drilled. I’m sure that a precise relation must exist between the type of the primer, the diameter of the flash hole, the type of propellant used, the shape of the cartridge case and the round caliber. I know that the ignition of the powder charge has to be fast, uniform and reliable. I assume that a larger diameter of the flash hole may accelerate the rate of propellant burning, obtaining a higher pressure. At the same time it must be a certain limit of the flash hole diameter and a small flash hole diameter also should have some advantages. I would like to hear from somebody who knows more about this interesting subject. — Note: Large caliber rounds [20mm and over] may have a flash tube which is normally perforated on the side. Thanks in advance for any help, Liviu 03/10/07



It’s true that flash hole size has a direct effect on internal ballistics and it must be designed with several things in mind. Primer pellet size and energy, powder type and charge, case size and configuration, and operating pressure, just to name a few. But with todays components pretty well established and standardized, ballistics engineers have a good handle on it. When something important is changed, such as the conversion from corrosive to NC primers, adjustments have to be made. But those things don’t come along that often and I believe you will see todays standardized flash hole sizes remain the same for some time.

Speaking from a shooters perspective, attention to flash holes is one of the small details that can lead to improved, or worsening accuracy. Most shooters will agree that flasholes that are uniform in diameter result in more consistent ignition and, therefore, greater accuracy. The exact size of the uniformed flash holes doesn’t seem to be critical so most will simply enlarge the vents to a size that will clean up the largest, within reason of course. There is some evidence to suggest that the length of the vent is more critical than the diameter so many shooters will also uniform that dimension with special tools.

All of this is done, keeping in mind that flasholes work in both directions. They transmit the high pressure gas from the primer to the powder charge but they also transmit the even higher pressure gasses from the case back into the primer. Right now, the consensus is that small, non magnum primers, and small flasholes produce the greatest accuracy.

In large caliber ammunition (artillery for example) the side perforated tubes are actually the primer itself. A flash tube is used to ignite the powder charge in the middle or front of the case and does not contain any explosive and is not perforated on the side.



Flash hole diameters do play a role in internal ballistics. Due to the nature (size) of small caliber (rifle/ pistol) cartridges, the size variations are limited. Among reloaders, extensive testing of small caliber cartridges has been made using flash tubes and it appears to have little or no merit in better and more consistent ignition. The manufacturing costs appear to outweigh the limited benefits. Interestingly, different types of gun powder layered in a small caliber cartridge appear to have some merit in better ballistics and accuracy. This is due to the pressure curve of the gases being able to be controlled. With current smokeless powders and the volume of space available in the small calibers the benifits appear somewhat limited.

Interesting subject, but internal ballistics give my gray matter a workout beyond their capabilities!




I think we’re gonna have to agree to disagree on one point. The concept of flash tubes and frontal ignition, and duplex loading (in small arms cartridges) is one that has been played with off and on for the past 150 years. Any resulting improvements in ballistic performance or accuracy has been anecdotal at best. The disadvantages are many.