compare ANY of these videos muzzle flashs to the ones in OP's vid http://www.youtube.com/user/freddiew
cheif and scorpion looked very cheap
nobody knew what a witch was
chainsaw and the time it took to saw through a tounge
Chief was an actual manufactured Master Chief dress-up costume, I believe.
The muzzle flashing is not quite the most important aspect of the film and to myself, wasn't even something that I focussed upon. I would only notice the quality of it compared to other films if I went back and looked for it, due to you pointing it out.
Scorpion was likely another party-style costume. It, alongside Master Chief would look cheap, since they're both made in a factory to be produced at a cheap cost and delivered to lots of stores and people. I don't think this film was very high-budget, so they weren't likely to spend a lot of time and money on making their own perfect costumes for characters that are shown for about 3 minutes.
While the chainsaw thing is true, I do believe it was in a way, parodying the use of the chainsaws in GoW to get through obstacles, which for example, it takes a lot of time to put the chainsaw through some very thin skin-mesh inside a worm. If not that, then it's there for keeping that shot there for longer to entertain.
It's not like L4D is that realistic concerning the smoker tongues when all it takes is a light butting from a gun to get one off. If it took that little effort, then no one would ever be tangled by them.
Only the witch part do I find to be of significance, mainly since I myself thought it unusual that they wouldn't know a witch when they hear one, particularly considering Bill is already dead. When I saw the witch, I first thought "They've met witches before. The opening cinematic to L4D even shows Bill finding one."