Flash Hole
The small hole in the bottom of a cartridge case that lets the primer's flash reach the powder. Most centerfire cases use one centered hole.
Most centerfire cases have one centered flash hole, drilled or punched to a standard diameter. A few precision-oriented brands (Lapua small-rifle primers) use tighter, more consistent flash holes that improve ignition uniformity.
Reloaders deburr the inside of the flash hole before first use. A small tool removes the punching swarf so the primer flash exits evenly, reducing ES on charge-sensitive loads.