Thanks to Marxman, when selecting which ammo type you want to run, it now displays ballistic gel tests for the internal ballistics! This should help illustrate the advantages and drawbacks of different ammo types, along with understanding the explosive trauma that'll be inflicted on your opponents. I'll also go through the basics of how generally different ammo types perform. (Keep in mind right now, these are a work in progress and haven't been done for every round)
Handgun rounds will generally do less damage and penetrate less armor than rifle rounds, but will have low recoil and high rate of fire in a compact smg package, or make a nice secondary for desperate times. Be aware you'll have trouble getting through lvl3a helmets and limb protection, and will suck against rifle rated plates. Depending on the armor your opponent has, you may want to magdump a few rounds in their pelvis to paralyze them, or, if you have especially good marksmenship, get a headshot.
Jacketed hollow points can be used to improve your terminal ballistics, expanding on contact, adding extra lethality while minimizing over-penetration. Using higher pressure 9mm ammo, or 5.7x25mm will reach those higher velocities for beating lvl3a armor, and upgrading to the fatter 45acp or 44 magnum will make meaner holes in squishy bits.
Shotguns have unrivaled performance against unarmored bits, but also don't have great performance against armor. Using the right load can let you spew a huge spread of shot, forcing enemies to keep their heads down, or a tight spread to rival rifles at range. Just make sure to aim at unarmored bits, like limbs, the groin, or face depending on the enemy's armor. Or you can use sabot slugs to rip right through non-rifle rated plates and pulverize anything behind.
Rifles rounds, like 5.56x45mm, can rip through helmets and limb protection and provide much greater accuracy and terminal ballistics than handgun calibers. You'll have a wide variety of options to choose from too.
Just looking at 5.56x45mm, ball ammo makes for a great jack of all trades, while soft points make for a close quarters machine, deforming on contact to create nasty wounds, but suffer when it comes to accuracy or armor penetration. Open tip match rounds expand less, but are great marksmen rounds when pinpoint accuracy is demanded.
This clips shows the devastating power of 5.56x45mm, but also shows that it can be stopped by lvl3 or lvl4 plates. You'll want to follow up your quick double taps to the chest with a headshot for a Mozambique drill or aim lower at the pelvis and let your recoil ride up for a Zipper drill against enemies who need extra encouragement to be sent to Valhalla. You could also use the spicier m855a1 or m995 to rip right through lvl3 armor.
5.56x45mm is no slouch against two legged targets, however, if you have a taste for the excessive or need something to guarantee a one hit kill at longer ranges, .308 maybe for you, at a cost of much harsher recoil. Ball is good, but soft points will absolutely pulverize any poor sob at the end of your barrel.
.308 AP, when you don't feel like worrying about anybody's armor getting in the way of your sweet, sweet certified juicy tango removal.
However, some of you may prefer a stealthy playstyle while rocking the accuracy of a rifle. 300 blackout subsonic ammo will keep you whisper quiet, for tricky flanks or round ending assassinations. However, your terminal ballistics won't be much better than 45acp, so watch out against plates, limb protection, and helmets.
5.56x45mm is great, but it doesn't do particularly great out of shorter barrel lengths. The supersonic 300 blackout polymer tips will have much better expansion out of a 10" or less barrel than 5.56x45mm soft points. This makes for a great tool for high stakes close quarters combat. The recoil however, will be a little more spicy.