well, actually petard would work fine with a British audience, but in the US you might do better with the unwieldy term "small homemade explosives known as "potato bombs"" that the AP used in describing Thursday's protests (see for photos) in Bogotá, and after that just use the term potato bomb. But if you just used the term 'potato bomb' alone from the get go, my guess is that most US listeners would think a potato was involved in making the bomb. Instead they look like a US style baked potato wrapped in tinfoil, but have gunpowder inside instead of a potato. Personally I think they are truly tragic strategy at protests, since they are used as an excuse for a brutal response from the police, as can be seen in this video of the papa throwing, and response, on August 21st at the Universidad Nacional. I went in the other entrance trying to meet a prof in the Sociology department, and about half way across campus a stream of people came towards me with their eyes watering. It was astounding how much of the campus they managed to fill with tear gas without being able to come in (cops are not allowed on campus). Classes were cancelled. Not exactly useful for building a broad based movement for justice and equality in my book.