Hiram Berdan had a real good design for fulminate of mercury primers back in the 1860s,
Made black powder cartridges of the time ignite MUCH more reliably.
(Hiram Berdan of Berdan's Sharps Shooters fame during the American 'Civil' War)
Although there are TONS of urban myth stories,
Sharps mass produced long range, BREACH LOADING, PAPER CARTRIDGE rifles at the time of the war,
Long brass rifle cartridges were new, and under powered, along with being unreliable.
Hiram Berdan worked with the department of the army & Sharps to develop a reliable metallic cartridge for long range rifles.
Hiram Berdan, Sharps (and others) are directly responsible for the long, drawn case for rifles (instead of cast, punched or turned),
They are responsible for using brass instead of copper, tin or zinc cases,
They are responsible for a separate pocket for primers instead of priming materials cast into case directly (rim fire),
The first varnish sealed cartridges (all weather) are a Berdan/Sharps invention.
The modern brass cartridge case we know now is a direct result of Berdan & Sharps.
Now, keep in mind that the innovations were worked out in advance, making Berdan's Sharps Shooters THE most effective, all weather, modern metallic cartridge units in history.
There wasn't any 'Experimentation' here, these guys were educated in industral processes, looked at ALL conditions & viarables the troops would be working under, and addressed those issues in advance.
The Army idiots later tried copper case cartridges (cheaper), which separated in the chambers rendering the rifle useless in battle, and screwed a bunch of other stuff up...
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The Berdan ignition system was devised for black powder, which drew moisture & liked to clog flash holes.
More flash holes, better chance of igniting the powder charge.
Two (or three) off-set flash holes (off-set from center line) simply don't lend themselves well to common Up & Down stright line reloading equipment.
'Boxer' or 'Stright' primed cases are the easiest to reload with commonly available equipment.
Today, with powder that has a moisture seal on the grains, and MUCH hotter primer materials, we don't have a need for the duplicated flash holes between primer pocket & case powder cavity, so it's not used much anymore other than countries that simply use it because of tradition, since it adds nothing to reliability of ignition with modern primers & powder.