Alright, let's get started...

1. First, open up Flash and make yourself a new file. Make sure you save often when you are working with Flash... I have heard too many stories of people losing their files because they didn't save often enough.

2. Set the background of your file to black. Black backgrounds are best for muzzle flashes, because there's so much contrast, it just makes it look more intense. Set the width to 550 and the height to 300. I just happen to like these dimensions, but you can set them to anything.

3. Create 3 layers: Gun, Muzzle Flash, Shot - order these as bottom to top, so gun's on bottom.

4. Place your gun of choice (I'm using Frommeyer's P90 gun template) onto the left of the stage area, with the back of the gun slightly off the stage. I know that the result above doesn't do this, but that's because I moved it. You can really place the gun anywhere you'd like. Make sure it's on the Gun layer.

5. Above is the end of frame 1. Move on to frame 2, and make a new keyframe for each of your layers.

6. So frame 1 was the set-up frame. Frame 2 will be the actual BANG! frame, where the gun goes off! Select the gun and move it back a tiny bit for the recoil. Zoomed in at 100%, I would select it and using my left arrow key, I would nudge it over anywhere from 4-10 times (pixels), depending on how big of a recoil you want. In real life, surprisingly, there isn't as much recoil as you might think.

7. Now, still in frame 2, go to the layer called Muzzle Flash, which should be your middle frame. This layer will be our base to the muzzle flash. It will be very broad and blurry, without much sharpness to the flash. Here's how to draw it. Take a very light yellow, almost white, and using the Brush tool and a big circular brush, draw a flash-like shape like in the example below. It doesn't have to be perfect because it will be blurry. Focus on not making it too big or small compared to the gun.

8. Once you're happy with your blob-looking shape, we're going to make it look more realistic and a lot cooler looking. Select the shape, and make it a Movie Clip. To do this, if you're unfamiliar how to, go to the top menus and go Modify > Convert To Symbol. We will name it bg_flash because it is the background of our muzzle flash.

9. After it's a movie clip, select it on the stage and give it a nice Blur Filter. View the image below to see how and what parameters. The blur amount on the x and y can differ depending on how you like it. But really, it shouldn't be too solid, yet the outside should be blured only a little. What I'm trying to say is use your best judgement to blur the flash, but don't over or underdo it.

10. We're now finished with the Muzzle Flash layer for Frame 2. Now we'll work on the layer named Shot. Select the layer and this will be the main gunshot flash. It will be a lot more defined and clearer in shape than the muzzle flash layer. Take your brush tool, and with a much thinner brush and a light orange color, draw some flash. Repeat with more orange, red, white, yellow colors until you have a result similar to the one below:

Once again, don't worry about it being perfectly shaped because it will be blurred.

 11. After you are done with drawing the darker parts of this burst-like flash, take the color white, and color the inside base part of the flash, as this is the most intense part of the burst. Continue when you have something like below:

12. Alright, we are finally getting somewhere with this! It's starting to look better, but there are still some more things. We are going to blur this burst flash, so make sure you're on the Shot layer and select the burst flash we just drew. Pretty much everything except the muzzle flash and the gun. Convert it to a movie clip named shot_flash:

13. Select the shot_flash movie clip we just made, and give it a blur. I've used the below parameters, but use your own personal judgement. Do not blur this as much as the muzzle flash layer.

14. If you feel like the shot_flash movie clip isn't correctly in proportion with the muzzle_flash movie clip, feel free to resize it the way you want. I made mine a bit bigger because I thought the gun was too big compared to it. Select the gun_shot movie clip to resize it using the free-transform tool.

15. For the last thing we'll do with the shot_flash movie clip, select it and change its brightness to 100%. If you don't know how to do this, select the movie clip and on the Properties Panel, usually on the bottom of the screen (if you can't find it, then go to Window > Properties > Property), and click the Color Dropdown which will let you select Brightness. Change it to 100% like below:

 16. Now your gunshot flash is looking good! To make this even more realistic and awesome looking, we'll give the gun a little reflection of light. Select your gun and give it a drop shadow filter. Make sure inner drop shadow is selected and use the below parameters to give it an awesome look!

  17. Congratulations, you have finished Frame 2! That was the hardest frame, but once you do this a couple times, you will just fly through these things! Below is what your timeline whould look like after Steps 1 -16.

18. Now on to Frame 3, but don't worry, it's a lot easier than 2. Create a new keyframe for each of your layers.

 19. Select your gun and move it back towards its original position. If you moved it back for the recoil 6 pixels, move it towards its original position 2-3 pixels. If you did 10 pixels, move it back 3-4 pixels, etc.

 20. Completely delete everything on the Shot layer. That was only good for one frame, so you don't need it anymore. Just select it and press delete. Don't worry, it is still in your Library of objects; you only deleted it from the stage.

21. Select the Muzzle Flash layer and select your muzzle_flash movie clip. Similar to changing the shot_flash movie clip's brightness, you will change its transparency to 30%. Select the movie clip and in the properties panel, go to Color > Alpha. In the picture below, I have Onion Skinning turned on, so that's why you see a little from the previous 2 frames. Don't think that the shot_flash movie clip is still present.

22. Alright, finished with Frame 3. Create a new keyframe for each of your layers for Frame 4.

 23. For frame 4, all you have to do is move your gun slightly back towards its original position again, about a pixel or two away, and then change the alpha on the muzzle_flash movie clip to 8%, so that it's barely visible.

  24. Here's what your timeline should look like now:

25. Congratulations, you are finished! Mess around with frames per secon rate until you are satisfied!

Experiement with different variations of shapes and distribution across frames to develop a style of your own! To view my shootest test I completed using this method, click the icon below. I changed the entire saturation of the animation to black and white, so that's why there is no color in it. Adding sound will really improve your gun shots, so that's another thing...