what's that thing mounted on the muzzle?
too big for a muzzle break, but too small for a silencer

Bakamealz said:

what's that thing mounted on the muzzle?
too big for a muzzle break, but too small for a silencer

It reminds me of a rifle grenade adapter, though again it's pretty small.

Bakamealz said:

what's that thing mounted on the muzzle?
too big for a muzzle break, but too small for a silencer

It's a flash suppressor. It reduces rise and flash, brings the sound down a little too in some cases.

This one looks like a KX3 Noveske specifically.