Berdan is the type of primer
used for mil spec small arms ammunition
. Almost all of the surplus
ammunition that is available in the bulk ammo
suppliers will be Berdan primed.
Compared with Boxer Primers that only have one flash hole, Berdan primed cases commonly have two smaller flash holes. This forces the hot gases generated by the ignition at a much higher pressure and supposedly makes for a faster and more reliable ignition of the powder charge.
In my experience, however, Boxer primed ammo is just as reliable, I have never have had a misfire in my reloads using Winchester Boxer primers.
It also has a reputation for being hard to reload but this is mostly because of modern reloading equipment which is almost always engineered to use Boxer primed ammunition cases. After all, in battle, where most Berdan primed ammo was meant to be used anyway, you don't really pick up your brass after shooting the enemy.
Modern pimers that are sold in shooting supplies stores like CCI and Winchester are Boxer type primers.