Firstly, study how real muzzle flashes look in various lighting situations. They will look different in daylight, compared to interior lighting situations.
I find the muzzle flash is brighter/whiter near the barrel tip, and gets yellowish or orange as it gets farther from the barrel.
To make the lighting in the photo brighter, start with lighting that comes from the sides, or further from the access of the camera. This will add more shadows, so you can brighten them in post, or you can have a light source glowing from near where the muzzle flash would glow. I use the Dodge Tool in Photoshop to brighten the tip of the gun barrel, the fingers near the barrel tip....Also, desaturate and darken the orange tip of your air-soft gun.