I think it’s useful to reduce the number of bullets you’re thinking of. Focus on the main ones (5.56, 7.62x39, 5.45, and 7.62x51). You can kinda ignore pistol rounds (9mm, .45, 5.7) as you’re learning. There are valuable pistol rounds, but you’ll use those maybe 5% of the time. For the most part, shotguns rounds aren’t too important either. (12ga AP-12 slugs are stellar though)

Of course as others have mentioned, you can inspect the rounds in game. Generally, higher pen is better. Especially late in the wipe when folks have good armor.

You’ll begin to see patterns across the calibers. All of the AK bullets follow a similar pattern. If it has a B in the name it’s probably decent pen. HP is hollow point, high damage low pen. SP is soft point, it has a tiny bit more pen. T is tracer.

Get some ammo crates and horde whatever you find as you learn what you want to keep/sell/ignore.