Mega Kill Clip is a trait in Destiny 2 that can appear on 22 weapons. An enhanced version exists on crafted and some newer weapons.
What Mega Kill Clip does
Reloading after a final blow grants a period of increased damage. Additional final blows before reloading further increases the duration of the damage period.
Enhanced version
Reloading after a final blow grants a longer period of increased damage. Additional final blows before reloading further increases the duration of the damage period.