Less than you would think. Armour-piercing ammo does not unlock until Career (Wardog) level 82 for 7.62x54mm and 83 for 5.56mm, and BULKHEAD expects the median player to be around level 20 after 20 to 25 hours. Early in a season almost nobody in your lobby has AP.
FMJ, the default round, stays the right choice for a long time. In range tests AP did less damage than FMJ against Light armour (18 against 21) and only edged ahead at Medium plate.
Legs are unarmoured at every tier. Against a heavily plated enemy, shoot the legs.
Where veterans do pull ahead is armour and better rifles. Put your money into armour and one good optic first; attachments carry across every weapon. Which rifle to buy is argued between creators, so read the rifle card in the combat chapter before spending on one.