Rotating gas-check

A rotating gas-check (more commonly known as an automatic gas-check) was a copper plate that automatically attached itself to a specially-designed studless projectile of rifled muzzle-loading ("RML") artillery, sealing the escape of gas between the projectile and the barrel and imparting axial rotation to the projectile.
External links.
1. Photos of a used 9 inch automatic gas-check recovered from the sea. http://fortlytton.net.au/?page_id=294
2. Photos of four gas-checks. http://www.victorianforts.co.uk/art/check.htm
3. Photo of 17.72 inch automatic gas-check http://www.bbc.co.uk/ahistoryoftheworld/objects/_GozUMNFTpem-NJXZpFgAQ