I second the .303 for a martini.

I occasionally handle and shoot a very nice Francotte Martini built around the .303 and it's a honey.
Slender, light, pointy -- with the express sights it's like holding a laser pointer, and it's also set up with QD scope mounts (although I've never shot it with a scope) for longer-distance shooting.
It likes the long bullets best, and with that combination it's perfect for almost anything.

--Tinker