When did the U. S. military start using plastic canteens?

The individual metal 1-quart canteen began to be replaced by a plastic version during the Vietnam era.

At the same time, a plastic 2-quart canteen was also developed and began to be issued as needed.

As a general rule, there are no situations where a metal can could be used becuase they are not issued anymore; however, that does not prevent an individual from privately purchchasing some sort of civilan metal canteen and using it - but no one wants to carry one.

In fact, with the development and issue of personal hydration systems, no one really uses canteens anymore in the US military.

Plastic Canteen

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RE:

When did the U. S. military start using plastic canteens?

Are there any situations in which they still use metallic canteens?

Right around the time of the Viet Nam war. Front line soldiers like the fact that they do not have to worry about a metallic 'clunk' when the metal canteen strikes something.