No, it's not singleplayer AND multiplayer! When your in the editor, making a multiplayer level, you can change over to singleplayer by going file>preferences. then near the bottom it says singlplayer and multiplayer with radio buttons next to them. choose singleplayer and you can edit more options with entities. Be aware that some changes revert and do not work in multiplyer. The main thing is that you can change the gun properties without modifying the .fpi
"There are very few problems in life that cannot be solved by the proper application of high explosives"