Smaller caliber guns would not normally have true AP shells, but would often have semi-armor piercing shells, providing limited penetration capability. In practice, however, the targets smaller caliber guns could be effective against would be better dealt with using HE shells. A large-caliber HE shell would have a much larger explosive charge and would do much more damage. This is reflected in the rules by rolling an additional number of times for the larger caliber of shells on the HE column of the Gunfire Damage Allocation Table. This gives a player a better chance to repair the damaged system - not a better chance to repair it in a single roll.
Some further explanation is necessary.
Thus the maximum stacking limit of four should not be exceeded. A merchant ship playing piece is used to represent up to four ships in the same hex. For stacking purposes, the playing piece is considered to be four individual ships. Merchant ships are attacked individually in the same manner as warships. No armor is listed for HE shell hits to affect.