You can play in first person - CHECK
The gameplay revolves largely around shooting things with guns - CHECK
It can be classed as an FPS
FPS Check
It has a linear game play like an FPS - no check
It has different levels like an FPS - no check
You just run around and shoot stuff - no check
RPG Check:
It has a huge world you can explore - check
you can go where ever you want to - check
you can pick up quests in every settlement - check
your actions will earn you negative and positive karma - check
you can pretend to be anyone you like - check
there are towns and outposts - check
it has player housing - check
you can use guns, swords, pipes, baseball bats - check
you can wear regular clothes if you want to - check
Oh and game play does not revolve around mainly shooting stuff...