I like the muzzle flash so far... lights up dark places in a... er... flash the effect is creepy with some zeds bearing down on you.
I'd like to see minor improvments if possible later on... for instance to begin with, shooting indoors could hinder hearing a bit with ringing if your ears if you're pumping rounds out continuously and also shooting in the dark could give a hazy image effect or frozen image for a split second after. The longer you survive and do this or better your skills if that's a thing somepoint the less this effect will happen or not at all eventually.
Maybe firing close to someone can deafen them for a short while?... too much?
I'd like to see the muzzle flash have effects on the environment like if someone is prone it'll kick up some dust/dirt or shooting from foliage will spread some leaves like confetti.