Megatron only has a split second to decide what he's going to do as Sentinel takes the extra moment to roll out of the way and aim.

Some of the shots bury themselves into his armor as he starts firing. The upper layers have holes burned into them where they land - exposing more vulnerable framing and energon lines. While the gunfire might have downed a lesser armored Decepticon, he finds himself able to continue to rush forward and attempt to give the Prime a well-aimed fist to the face as hard as his arm could muster.