cylinder, pipe, tube, gun barrel, sleeve
筒 centers on a hollow cylindrical shape: a tube, pipe, or cylinder. The gun barrel and sleeve senses are specific applications of that same tubular form.
筒 combines 竹 (bamboo) with 同, which likely contributes the sound. Bamboo tubes or pipes are a natural source for the meaning of cylinder or tube.
Bamboo 竹 is naturally hollow and tubular, and 同 reinforces the idea of a uniform hollow shape. Picture a bamboo stalk cut into a straight cylinder or pipe.
For トウ, imagine a toe stuck inside a narrow bamboo tube: toe -> トウ, and the tube is just wide enough for a toe.
pipe; tube; cylinder
envelope
canteen; flask; water bottle; thermos
cylinder
round well
cylinder
being heard (by others); being overheard; being leaked (of plans, secrets, etc.)
well curb; well kerb
container made by cutting a piece of bamboo lengthways; bamboo tube; bamboo cylinder
(warning) flare (esp. used on roads and railways)
quiver
tube for shaking and dispensing lottery sticks
padded envelope
rocket launcher
barrel printer
cylindrical projection
cylindrical
photographic zenith tube
single barrel; single-barrelled
cardboard mailing tube; shipping tube