While the AC-20 might do much more raw damage, (it can destroy some light mechs, like the Commando in one shot) but you do not get much ammo per ton, so a lot of your mech's SPEHSS* and tonnage will be dedicated to that huge gun, not leaving much room for other weapons and armor. It also has a very short range, so you will often find yourself lacking your primary source of firepower when fighting...say...A Catapult.
The AC-10 might only do half of the AC-20's damage, but it is quite long ranged, meaning that you will not be so defenseless against THE MIGHTY CATAPULT. Also, unlike most other long range weapons, the AC-10 has
no minimum range, (in the board game, at least.) which means that you can use it to blast the enemy's face off at short range AND long range. Wait, did i forget to mention that the AC-10 lets you hve more ammo per ton?
Simply put:
AC-20
-Enough firepower to compete with a Gauss Rifle
-Short Range
-Easy to run out of ammo.
-Quite heavy.
AC-10
-Firepower that matches a PPC.
-Good range.
-Plenty of ammo per ton.
-Still kinda heavy.
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