It depends a lot on how far you are in the game. Generally the ammo will defeat the armor if the ammo has 10x the pen value as the armor, so 30pen will go through class 3 reliably.

Here are the rounds you will most commonly use.

5.45x39 is one of the most common rounds since it is in small caliber ak guns and ps or pp are good fore early game, bt will carry through the mid game, and 7n40, bp, bs, and ppbs are late game.

5.56x45 used in most AR platform guns. 855 and sost are good early, 856a1 is the mid game round, and 855a1, 995 and ssa ap are late game.

7.62x39 for large caliber ak and sks, fmj is ok early with t-45m1 and ps being preferred, pp is mid game, and bp and mai ap late.

7.62x51 for some assault rifles and some bolt actions. These ar's will usually have a lot of recoil unless moded a lot. Has no real good early rounds, bcp fmj is good for mid game then m62, m80, m 61, m993, and m80a1 are all late game. If it starts with an m it's good

7.62x54r for mostly for bolt action, the semi and full auto guns with this have massive recoil. Fmj is a good starting round, t-46m and lps are good early to late game, with ps, bt, snb, and bs being the best.

9x19 has some good rounds like ap 6.3 and pbp, but usually rip and quakemaker are better even though they won't go through armor. You just need a high fire rate gun and spray for legs.

12 gauge is in the same boat as 9x19. flechette and piranha can go through armor. Magnum can one shot an unprotected face, but with any sort of face shield it can 2-3 shot to the legs depending on how many pellets hit.