The rebuilds tell the story
The ones that were used in combat probably were shot to pieces, these have new barrels. The ones that were carried by cooks, staff officers etc, have barrels that are like new. I would say that the large percentages were not fired much since an awful lot of carbines have their original barrels and these barrels have a lot of life left. Additionally the carbine can shoot a lot of rounds and still be in great shape. Some of the 6,000 shot test carbines were simply checked out and sent for issue after the test. If you compare the carbines to the average field grade Garands, there are a lot more shot out Garands and almost all WW2 built rifles have new barrels. The service life of the Garand barrel is probably less than half of what the carbine will do.
You have to view it all from the perspective that ALL carbines and almost all spare parts were made in a 3-4 year period and they never made another, yet many are still around. The Garand was resumed in the 50s and a lot more made until 1956.