Obturating ring

The term "O-ring" is sometimes used to describe this kind of pressure seal.

Obturating rings are common in artillery, where the steel or cast-iron casing of the shell is too hard to practically deform to provide a tight seal for the propellant gases.

An obturating ring which is called driving band made of a softer material is the standard solution for that problem.

Mortar bombs also use obturating rings to provide a seal around the projectile.

[clarification needed] Another obturating ring may be used on sliding/falling breech-blocks from the opposite side of the chamber to provide a tight seal there if the charge is bagged and lacks a case (examples include early Krupp guns to Royal Ordnance L11 to M777).