There are a couple barrels out there that are not much bigger than a soda straw.
Most of the guns that use the very light barrels use a breech block around the chamber area.
I'm thinking of guns like the Bronco 22, and similar.
I suspect that much of the material is used to provide the paying customer with the feeling that they are getting something for their money. Some also to make the rifle feel like it should, balance-wise.
Some is just to keep the barrel from being bent from being wedged in behind the truck seat with all the other junk.
Take a look at the old Stevens tip-up pistols. The barrels on a lot of those would be about the size you speak of, and they had cutouts and grooves, for the extractor and latch, right through the same area as the chamber. Not modern, and few lawyers were on the list of folk that had to sign off on the design...but it shows what was done.
Cheers
Trev