A bison tube is a small, metal, water-tight cylindrical container that can be used for micro-caches. These containers are usually small enough to fit on a keychain, and were originally designed to hold pills. The name is derived from the company which manufactures most of these types of tubes, Bison Designs.
Available sizes include:
2 3/8 x 5/8 in / 60 x 16 mm (Small)
2 3/4 x 5/8 in / 70 x 16 mm (Large)
2 x 3/4 in / 50 x 20 mm (Scuba Tank or Micro-Cache Capsule)
3 x 1 in / 75 x 25 mm (Extra Large)
from Cacheopedia
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