The AC 20's massive slugs cause tremendous damage on impact, and can even destroy some scout 'Mechs with a single hit. However, the AC 20's legendary damage begins to decrease beyond 270m. Even so, the ballistic maximum range profile means that it can still deal reduced damage up to two times its optimal range (up to 540m).
All this firepower does come at a cost. The weapon itself weighs a whopping fourteen tons and takes up ten slots of space, making it the largest weapon in the game. Even with the presence of an appropriate hardpoint, its 10-slot bulk prohibits it from being mounted in the arms of almost all 'Mechs, and the use of an XL Engine will prevent it from being mounted in a side torso as well. Its ammo is similarly heavy; each ton of AC 20 ammo contains a measly seven rounds.
Very few 'Mechs can mount the AC20 in an arm location. The hand and lower arm actuators must not be installed to free up enough room for the weapon. Some notable examples of 'Mechs that can mount an AC20 in the arm are the Centurion CN9-YLW, Raven RVN-4X, Highlander HGN-733C, the UrbanMech, the JagerMech, and all versions of the King Crab. Note that the absence of these same actuators also mean that these 'Mechs cannot move their arms side-to-side (0° horizontal arm movement).
-Gauss Rifle: Has a similar DMG rating but can reach out farther.