To use acid, explosive, flame, electric, and flash rounds you have to combine the ammo to the Grenade Launcher.
As for Magnum rounds, they aren't that rare in the game. There are 2 ways you can get them real easily. #1 is to buy them in buy section, they don't show up, but you can get the ammo just by purchasing a magnum and then selling the gun and you will have magnum ammo. #2 is to abuse the Chapter 3-1 in the game for it as seen in this youtube video: [youtube]94_fpwHagIU[/youtube]