I joined about a week late and im thinking there wouldnt be enough time to make a proper full fledged game.

Would a demo of let's say 10 rounds of a shooter be enough?

Same, just do it cause even if you dont win you still made something and thats good too :D

Doing a full fledged game is not a requirement of any game jam, as far as I know. Go ahead! Build a prototype of what you wanna achieve!

other game jams only give you a weekend to develop so 7 days is not that bad

The jam was long to let many people participate, not to dedicate 14 days of your life to the jam, so yes it is totally fine and valid to make what you consider to be a demo.