Blender is free and it's my favorite editor. There are a bazillion tutorials and a wonderful community to help you get started. I use it for Windows, but it's available for Mac as well. Creating gunshots from scratch is hard, but Detonation Films has some decent free images that you can easily composite into your movie in Blender.
HitFilm Express isn't too expensive and has a lot of nice effects, including muzzle flash presets. It's very easy to use and there is a free trial. I've also heard good things about DaVinci Resolve Lite (it's free), but I've never used it myself.
If you need gunshot sounds, Audacity is a good, free audio editor. NCH WavePad is another free one that I recently downloaded, but haven't used on any projects yet.