An assembly for effecting a variable volume chamber for use in a cannon having a breech assembly operably coupled to a chamber, and includes the chamber having a certain size being partially defined by a constant diameter for a known depth dimension and having a known volume and a breech plug assembly disposable in the chamber to a selectively variable depth, the depth being less than the known depth of the chamber such that the plug fills a first selectively variable portion of the known volume of the chamber, thereby leaving a second variable portion of the known volume of the chamber free and defining a chamber variable volume. A Cannon including the variable volume chamber and a method of effecting a variable volume chamber are further included.