Field Catalogue rules are hit or miss.
As regards armour piercing, it's dependent on muzzle velocity, size of ammunition, and a minimum hardness of the ammunition, which isn't covered in the book.
Greater muzzle velocity, plus increased size of ammunition, means greater recoil.
Then, you can configure the ammunition itself to have greater penetration, which would be either discarding sabot/elongated dart, jet of molten metal, or a very hard/heavy cone.
Easier at one hundred twenty millimetres, than twelve.