Without budget you can always choose to rotoscope it in the frame with photoshop.
Watch a movie, freeze frames and look how it looks.
In my latest project I added photoshop-drawn muzzle flashes.
I also used a flashlight to add lighting and it was a frontal shot, so my painting-skills are barely visible
That's a good idea, but doesn't it require drawing skills?
No.
WalterB makes a great point that you just need Photoshop!
I did this for one of my 48 Hour Projects. I didn't know After Effects, at the time, so I would go into Premiere and highlight the supposed gunshots video with the Work Area Bar, for about 5 frames. I output those frames as a TIFF SEQUENCE and opened up the 5 TIFF frames in Photoshop. Then, I opened up a picture or framegrab of the explosion from TERRARIUM, seen at the 1:25 mark
HERE. I used the CLONE TOOL and with a small circle stamp, selected the fiery part of the explosion and started stamping/drawing the flare of the gun on 3 of the 5 gunshot frames (frames 1, 3 and 5). That way the machine gun flared on and off. I also drew (stamped) each frame with different shapes/sizes.
I also used the DODGE/BURN Tool, with a big circle stamp, to brighten up a fairly wide area around the gun, so it looked like ambient brightness from the gun flash. I IMPORTED the frames as a TIFF SEQUENCE back into Premiere and just put them on the video layer above the original footage.
If you just look at the still, it's really crude. But, in the context of a moving shot (4:41 mark, in the video below) it looks passable. The shot where
I'M ALL BLOODY and I shoot Veronica, looks really good and all I did was CLONE and DODGE one frame (6:42 mark, below). Amazing that a single frame can have that impact.
https://www.youtube.com/watch?v=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/qWLtdhqZ-3w&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/qWLtdhqZ-3w&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
In EXILE, there is a scene in a dark cave, where Reyna shoots. I used WalterB's flashlight trick for that! I don't have that clip posted, but I was able to put a lot more time into realistic gunshots, for that movie, so I combined shots of NC Cinema gunshots (in After Effects), on top of Visionlab Studio's muzzle flash animation. That looks pretty real, because of the smoke included in the NC Cinema footage (or any footage of an actual charge going off). You can see those kind of smokey gunshots, starting at the :55 second mark, in this scene:
https://www.youtube.com/watch?v=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/02tSCbnJPA0&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/02tSCbnJPA0&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>