Not really "obscure", but I enjoy things that are janky, stupid, or cursed, so: FN 5.7x28mm. Invented to defeat mythical Soviet paratroopers with titanium body armor, mostly used by warrior cops and the Stargate program. What makes it an example of cursed engineering is that FN really really wanted that long, totally straight magazine on top of the P90 so they designed 5.7 with no taper whatsoever. This makes extraction very challenging, so their solution was to just coat every single case in a layer of slick plastic, in a classic example of "so what if it's twice as expensive, we're selling these to NATO."