I rolled up the poster and put it in a tube.
They carried water in a bamboo tube.
gun barrel; barrel of a firearm
A firearm or cannon barrel viewed as a hollow tube. In modern precise language, 銃身 or 砲身 is often clearer, but 筒 can appear in older, descriptive, or less technical wording.
You must not point the barrel of a gun at people.
Old-fashioned or historical use in which 筒 refers to the firearm or cannon itself, not just the barrel. Modern Japanese normally uses 銃, 鉄砲, or 大砲.
In old records, 筒 can refer to a gun or cannon.
well lining; well curb
Obscure construction-related use for the cylindrical lining or curb of a well. It is limited and much less common than the general 'tube' meaning.
A well's 筒 is the lining or curb that supports the inside of the well.
I store the papers by putting them into a tube.
He pointed the gun barrel at the enemy.
quiver