Keepers? That works for me. They vary in shape and prominence, from barely there, to quite prominent and fitting into a squared recess in the barrel rim (an example of the latter is on a Dougall Lockfast). They might act as a gas seal or gas deflector (to reduce the size of the pin hole), or simply to 'round' the pin hole on the barrel, which would otherwise have a 'D' shape.
The Breechloading Armoury gun covered earlier, and seen again here, lacks them entirely, but it was built as a dual-fire gun. I can understand conversions from pinfire to centrefire having the 'keepers' filed off the standing breech, as the pin holes are generally filled during the process. There is no point in doing so for a dual-fire gun, so I have to speculate the Breechloading Armoury gun never had them at all, which is unusual for a pinfire.
Thanks for your posts, gentlemen, I would never have gone back and noticed the difference otherwise!
