Definitions for "Obturation"
The momentary expansion of a cartridge case against chamber walls which minimizes the rearward flow of gases between the case and the chamber wall when the cartridge is fired.
This is when a cartridge expands in a firing seal to prevent it from happening.
The process of sealing the bore to prevent the escape of gas. Pistol bullets typically obturate when fired by expanding at the base.
Keywords:  stopping, act, opening, closing
The act of stopping up, or closing, an opening.