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I read this story around 1975. The main character traveled with some traders to a planet that had no metal, and that was being prevented from having any metal (I don't recall why). The planet's inhabitants were very ingenious in creating technology without metal. The part that sticks in my mind is when the main character went to a market with one of the traders, where they were attacked by the locals. The trader had obtained a local firearm which was almost entirely made of clay. The firearm's "trigger" worked a pump similar to a water gun, which built up air pressure inside the firearm. The firearm contained powdered coal, and striking a spark inside the pressurized firearm ignited the dust and propelled a dart.
I don't recall much of the plot, but I have always remembered that description of the firearm.