imm Largo is exactly what the name implies: its a long 9mm. 9x23mm to be exact. The guns that shoot this caliber are few and far between, including most notably the Astra 400, Destroyer Carbine, and a few pistols from Star.
9mm Largo is a completely obsolete cartridge. No one currently manufactures loaded ammunition for this caliber. Some poepl ehave reformed brass from other calibers to work in 9mm Largo barrels and others acquire and shoot surplus ammo.
The surplus ammo on the market right now is stuff made by the Santa barbara Ammunition Factory in Spain in the 50s, 60s, and 70s. The earlier stuff is corrosively primed. The Largo ammo I've seen has had extremely hard primers. Even after being shot, the primers don't show a dent. The primers do have a mark on them, a scuff on the metal, but they aren't indented like the primers of any ohter ammunition I've ever encountered. I assume the Largo ammo I've been exposed to was designed for use in machine guns because machine guns often are fed ammo with hard primers to prevent a double-firing condition.
As with any surplus ammo, there are bound to be some click bangs and duds. This was apparent with some of the Largo ammo I've shot, but one vintage in aprticular caused more trouble than others. I assume my friend got a bad batch or the ammo was stored improperly sometime during its 40+ year existence.
I like 9mm largo, but there's really no reason to have it. Other calibers have come out such as 40 S&W, 10mm, and .357 Sig which take the place of this obsolete caliber.