I was just wondering how I'd be able to get a rare weapon?

From the rare crate or from the elite soldier. Crates can be found around the maps and elites spawn randomly.

Also you need required amount of XP to use them.

Quite a lot of veteran players give them away every now and then too, and most will share if You ask politely. Only gun i don't give away freely is the Barrett and sometimes the VSS (when i'm low on them), because they're just too hard to come by.