Well, one of the simplest things you can do would be to use a cap gun, although this doesn't exactly do the trick for a muzzle flash. Also, if your character is firing an uzi, you'd go through a lot of caps, and where does one find a Uzi cap gun, anyway?
Another thing to do it film your actors, stick them in the computer, and use Ulead or Premiere to draw in the flashes, frame by frame.
You can also get a gun that fires blanks, but let's be realistic. This is a LOW BUDGET sfx page, and also, a lot of people viewing this are probably minors. Getting a gun, whether it fires blanks or real bullets, is a hassle.
The only other alternitive would be to use old flashbulbs.If you have a pistol other than an expensive airsoft gun, old flashbulbs make great muzzle flashes. Find some old "Magic cube" flashes -for the old Kodak instamatic, take 'em apart and wire 'em up - four muzzle flashes, no digital time wasting. They may melt a plastic gun, though, if you don't use some kind of shielding, like a cardboard collar.
