Most weapons have damage drop-off starting at 10m, meaning until 10m they do maximum damage, after that they linerarily do less damage the further away the target is. Soft-point ammunition increases the range at which they still do maximum damage to 15m but also decrease the distance at which they still do more than minimum damage by IIRC 5m. It's a straight up buff to very short ranges but lowers your DPS at longer distances, so it's the preferred choice on close quarter weapons like SMGs.
HVA increases the speed of your projectiles so they have less vertical drop-off at range and require less leading of targets, making it overall easier to hit targets with single shot or burst fire. However, to compensate, HVA increases the recoil and so makes the weapon harder to handle, reducing it's performance at ranges where you would prefer full-auto. It's the preferred choice for long range weapons. The exact speed and recoil difference is different on each weapon.
Slug ammo damage depends on the shotguns but is always significantly lower than the damage you would do if you hit an enemy with all pellets. I do not remember the exact values, but I think it was 500 for pump action and semi-auto shotguns, 400 for the full auto shotguns. Slug ammo still does a lot of damage, so if you can compensate for the high drop, it's theoretically possible to use them as mid-range sniper rifles or alternatives to battle rifles.