For the muzzle flash you can use a transmapped prop, parented to the gun. Have it scaled down to a minimum to start with, and if necessary move it back into the barrel of ther gun, in the frames where you want the flash, move it out of the barrel and scale it up again, then scale it back down and move it back into the barrel to fade the flash. It's best if you place the origin of the prop on the edge that meets the barrel, you should do this imediatly after loading the prop, before applying any transforms to it.

In the attached annimation I used the 'Jet Exhaust' prop which I picked up in the Free Stuff some time ago.