Gungame.io is a great shooting game where you start with a few guns and keep adding to them as you complete levels.
How to play
In a gun game, it's smart to try to get the other players to follow you and shoot backward. This is strong because it makes it hard for their bullets to hit you when you're going backward, but it's easy for you to hit them with your own.
To play, use the WASD keys to move your character and the left mouse button to shoot other people. You can also scroll to change guns.
Have fun and good luck!