rear of barrel vs rear of tube

rear of barrel and rear of tube both are nouns.

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

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

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

Other synonyms of rear of barrel include breech.

rear of barrel (noun)	rear of tube (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