OK, here is my block design for an action using the necked-up
(pardon my mumbling, that is how I talk with my tongue-in-cheek):
dark grey is receiver
light grey is breech bolt
magenta is cartridge chamber piece
and blue is barrel.
Obviously the feed is complicated by having seperate two
piece cartridge chamber to inclose the necked up round
for insertion into the breech of the barrel.
Consider this: in the 40mm grenade round, the powder chamber
is about the size of a ten gauge shotgun shell. Most of the
40mm grenade cartridge case is metal to bring the outside
diameter up to the diameter of the projectile. This necked-up
cartridge system might have an application, although I
admit I did the sample cartridge purely as a joke.