The Broadwell Ring is a small ring found in French and German artillery. It sits in a recess at the rear end of the bore, and abuts against a flat plate. When the gun is fired, the rapidly expanding gases produced by the combustion of gunpowder force the Broadwell Ring against the plate, sealing the end of the bore against escape and directing the full force of the explosion toward propelling the bullet.
The Broadwell Ring is readily removed and replaced for maintenance purposes.
References
Broadwell Ring Wikipedia(Text) CC BY-SA