KanjiJLPT N2
Cylinder

cylinder, pipe, tube, gun barrel, sleeve

Mnemonic

Split a bamboo stalk and notice every section is the same — a natural cylinder. Same hollow center, same round walls, same smooth bore. Bamboo is nature's assembly line, pumping out identical cylinders one segment after another.
Additional thoughts
Bamboo is literally made of repeating cylindrical segments, all the same shape and structure. The kanji mirrors this: bamboo on top, same on the bottom, forming a cylinder. Think of looking through a bamboo tube — every cross-section is the same circle.
Quick recall
Bamboo segments are all the same — natural cylinders stacked end to end.

Details

The keyword for 筒 is cylinder. This kanji refers to a hollow, tube-shaped object, such as a pipe, tube, or cylindrical container. It evokes the image of any elongated, round vessel through which something can pass or be stored. The character is commonly associated with items like bamboo tubes, gun barrels, or mailing tubes.
On'yomi
とう
Kun'yomi
つつ