How much does our highway gas mileage decrease in a heavy rain? (Thanks to Alex Godunov of Old Dominion University for suggesting the question.)

Guns have long barrels to take full advantage of the force of the expanding gases to propel their projectiles. What would be the initial velocity of a bullet if the propellent in its cartridge exploded outside of a gun?

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Question suggestions are always welcome!

For more Fermi questions and answers, see Guesstimation 2.0: Solving Today’s Problems on the Back of a Napkin, by Lawrence Weinstein (Princeton University Press, 2012).

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