I often find these metal cylinders in the streets of London: a silver-colored tube with a round tapering at one end and a hole with no thread at the other.
What are they?
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These containers are nitrous oxide canisters
https://www.talktofrank.com/drug/nitrous-oxide
https://en.wikipedia.org/wiki/Recreational_use_of_nitrous_oxide
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1$\begingroup$ I would have guessed a CO2 cartridge used in a pellet gun, but a web search shows that it's more likely sniffers(?) debris from inhaling NO2. Thumbs up for Mr. D. Duck $\endgroup$ – fred_dot_u Apr 28 '19 at 23:19
