Facepunch just released the extended magazine into the game, which adds 25% to the capacity of ammo that it can originally hold

With Custom Magazine's, you can now add several types of extended magazines! By default, you can now also add 50%, 75%, and even 100% capacity increases! At 100%, this will double any weapons capacity! You can change those capacity increase percentages in the configuration, remove them, or even add your own! You can configure spawning of these items in standard Rust crates!