Experienced the same. Muzzle flashes are very dim.

In twilight for example they seem like their opacity is reduced or sth.

On the other hand - the muzzle flash from the little birds minigun seems fine.

Make sure to set your editor time to something after 21:30 and test again.

All dynamic light sources get completely crazy between 21:00 and 21:30.
This is described in issue 0003119 and 0002929.

This happens to me, too. Muzzle flashes and explosions are very weak in the night.
Whatever happened to the Muzzle flashes that were showcased in the videos that illuminated the environment so brightly? There's also a older screenshot of a muzzle flash here: http://media.pcgamer.com/files/2012/06/arma3_e3_201231.jpg

While it's on the screenshot not illuminating the environment, it's in a nightly setting and has full brightness and even a cool particle effect that we do not have in the Alpha. So since I don't know if Bohemia just changed their mind about them or if the current ones are just placeholders, I will support this ticket until we get a fix or if they are placeholder, until we get the real effects.

This was fixed with the recent (and ongoing) lighting overhaul.