If it's 300 grains, that's between half an ounce to 3/4ths an ounce of black gunpowder; not much, but enough to get a good fire going.
If it's 300 grams, that's about 10 ounces, and enough to get a NASTY fire going.
Unless black powder is under compression, however, there won't be an explosion.
And furthermore, this had better be a game question, and not a real-life one, or you'll get somebody hurt or killed.
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