As far as I know no one has come up with and oring that will place the in a fill valve for a gas gun or grenades shell. they are just so darn small as seen here
well I have found a ring that fits and you can get it at you local hobby shop!
These work! both in my pro arms 96 rd Grenade shells and in my kjw pistol
also while searching for the right sized oring i found they have the right sized ball bearings for my shells at ACE