This is a word not often used in the ballistic lexicon where modern rifle bullets are concerned, but where you are shooting a muzzleloader, shotgun or a gas checked lead projectile it has a point of conceptual understanding and its benefits are huge!

Cast Bullet Gas Checks below.

https://detroitammoco.com/gas-checks/

I came across the word obturation when examining the using soft lead, copper or gas checks where the bullets base (sometimes concave) expands to fully engage the rifling in the barrel thus preventing the escape of gas around the slightly undersized bullet.

In the case of my TC 50 cal muzzle loader, I use PowerBelt™ Bullets with a synthetic concave gas check base which obturates, seals the gas, as the slightly undersized lead bullet exits thus providing maximum power and accuracy as the lead does contact the rifling. And it loads easier than plastic saboted bullets. See the story below.

When the muzzle-loader is fired the synthetic base expands providing the obturation (gas seal) to prevent gas to escape around the slightly smaller diameter bullet and thus increases the muzzle velocity and energy delivered. This the same concept for shooting a shotgun with bird-shot or a rifled slug/wad where the wad or plastic base obturates (seals to prevent gas from escaping around the pellets) upon ignition.

https://en.wikipedia.org/wiki/Obturation

Good Shooting!