from The Century Dictionary and Cyclopedia
- That stops or plugs up; used in closing or stopping up: specifically applied to a primer for exploding the charge of powder in a cannon, and at the same time closing the vent, thus preventing the rush of gas through it in firing.
Sorry, no etymologies found.
Then, to stop this "erosion," the obturating (sealing) primer came into use.
When you called a "mortar bomb rubber ring" an "obturating ring" ...