rear of tube vs rear of barrel

rear of tube and rear of barrel both are nouns.

Word	Noun	Adjective	Verb	Adverb
rear of tube	Yes	No	No	No
rear of barrel	Yes	No	No	No

As nouns, rear of tube and rear of barrel are synonyms defined as:

rear of tube and rear of barrel: opening in the rear of the barrel of a gun where bullets can be loaded

Other synonyms of rear of tube include breech.

rear of tube (noun)	rear of barrel (noun)
opening in the rear of the barrel of a gun where bullets can be loaded	opening in the rear of the barrel of a gun where bullets can be loaded