1. The red coloured background on certain shots tell you that your current gun is capable of Rapid Firing or Crouchfiring your shot. Light Bowguns Rapid-Fire, Heavy Bowguns will Crouchfire.
2. When using a Heavy Bowgun, unsheathe the gun and press X + A to enter Crouchfire, it'll only allow you to enter Crouchfire if you have the right shots.
You can also enter Crouchfire by evading and during your roll press X, you'll "roll' straight into the Crouchfire position if you have the right shots with you, and it's slightly faster compared to the other method.
Light Bowguns will rapid fire automatically if you fire a shot that can be rapid fired.