Hmm...
1787: Charles Bladen discovers Mercury (II) Fulminate, a powerful primary explosive. He experiments with using it as a propellant, but the resultant explosion violently destroys his test weapon.
1789: Bladen and two of his old army cronies prove that the explosion of Mercury (II) Fulminate can trigger a secondary charge of gunpowder, and before the end of the year have built a test weapon. The cost is considerable, however, and nothing comes of it beyond a few demonstration weapons for Royalty.
1796: With the Revolutionary Wars hotting up, the "percussion lock" garners attention. Howard takes up the project, and proves that the percussion lock allows a weapon to fire more reliably during the rain. Suggestions it should be taken up are demurred owing to the fantastic costs it would involve.
1799: Ezekiel Baker's rifle is considered for the Rifle Brigade, and is produced in two versions - one with a flintlock, the other using the percussion lock. The percussion lock version is shown to be more reliable and accurate than all other competitors, and is adopted - though the design of the rifle is amended so the hammer can be replaced with a flint if need be and the frizzen is retained.
1802: Percussion cap production is holding steady, and it looks as if it will be possible to provide approximately two hundred to three hundred caps per man in the field as part of the rifle accoutrements - replacing the fine powder-horn.
1805: The Rifle Brigade goes into combat for the first time. The results achieved are noteworthy, and their reliability makes them a fine asset to the British Army.
1807: The French voltigeurs begin to use their own "fusils de percussion" in some elite units. As with the British, the production capabilities of the still-new device are the primary impediment to widespread deployment.
1820: P1819 Percussion Musket adopted in the United Kingdom, making it the second Great Power to switch to a standard-issue percussion musket. The others follow in short order.
(Doesn't set the world on fire, but probably changes the Sharpe books.

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