I feel like they put armor-piercing on the majority of automatic receivers to help mitigate the damage reduction, but how much armor do these receivers actually bypass? 30% like the legendary modifier?

Don't know specifics either, but my fully modded auto armor piercing assault rifle is mowing down all the Commonwealth scum! I love VATS, but i find i drop enemies even faster outside of VATS with my assault rifle.

According to the Better Weapon Descriptions mod, all the items that I had that can get an Automatic Piercing Receiver get 30% Armor Pierce. I checked all the Ballistic weapons I had, and the 10mm Pistol, the Pipe Pistol, the Combat Rifle, the Combat Shotgun, and the SMG all had the same values for all levels of Automatic Piercing Receivers.

Thanks for the info. So based on the wiki notes for the Penetrating legendary modifier (which is also 30% piercing):

The Penetrating mod's effectiveness, due to the exponential curve of armor based damage reduction, does not vary with the target's armor after a certain threshold. When the target's armor rating is less than 15% of the weapon's damage, which is very little, the damage is applied in full (meaning 99%) and the Penetrating effect wasted. If the armor rating is between 15% and 21.4%, a Penetrating weapon will still do full (99%) damage while a standard variant will already experience damage reduction, making the Penetrating weapon increasingly better by comparison (0% to 13.9% more damage). Past this point, which is still very low for semi-automatic weapons, having the Penetrating mod means a static 13.9% increase in effective damage.

I guess 14% is the number to keep in mind when comparing armor-piercing receivers to stronger non-piercing automatic receivers. Unfortunately, with all five ranks of Rifleman, semi-automatic rifles get 30% armor piercing all around, so any "edge" automatic rifles could have over semi-auto won't come from armor-piercing potential.

Where automatics do shine, though, is with legendary modifiers that benefit from a greater rate of fire. Especially Explosive and Wounding.

Think wounding ignore armor and explosive is an explosive effect who I don't know how is calculated.

Have an bleeding minigun, against minelurks i just have to hurt them a bit and they bleed to death. I guess most of the bullets 8 damage is absorbed but then you get 25 bleed * 20 bullets