The Magnum is the 10th and last of all the weapons which comes with 20 ammo, 2 upgrades can be bought: Ammo increasement and movement speed.
The Magnum delivers medium damage, a medium firing rate, and 20 ammo, which can be increased with an upgrade. There is an upgradable version called the Rail Gun, which delivers medium damage, a medium firing rate, and comes with 20 ammo which can be further upgraded with an upgrade.
It's recommended to buy this weapon once enough cash has been collected after the Plasma Cannon . The extra damage from the Magnum combined with Plasma Cannon ensures (most of the time) a OHKO. Both upgrades should be bought, but it's possible to opt out the Ammo increasement if you can live with it. The Rail Gun is not necessarily recommended since it's the same weapon with unseen damage increasement.