Long post here, but hopefully useful
OK, it works like this.
Say you've got 5mm JHP, and 5mm AP.
5mm JHP:
Damage mod: 2/1
Damage resistance mod: 35%
The base damage of an Assault Rifle is 8-16 per shot. If you load in JHP ammo, that's doubled to 16-32. However, your enemy's resistance is increased by 35% against the shots.
5mm AP:
Damage mod 1/2
Damage resistance mod: -35%
Once again, the base damage of the AR is 8-16 per shot. If you load in AP ammo, that's halved to 4-8. However, your enemy's resistance is decreased by 35% against the shots.
AND, in something not indicated anywhere, AP Ammo
completely ignores Damage Threshold.
So, against an unarmored opponent...
The most damage a burst of JHP can do is 83-166.
The most damage a burst of AP can do is 32-64.
Seems pretty bad there, doesn't it?
Then against Metal Armor...
The most damage a burst of JHP can do is 34-78.
The most damage a burst of AP can do is 32-64.
Getting closer.
And against Combat Armor...
The most damage a burst of JHP can do is 22-54.
The most damage a burst of AP can do is 30-61.
AP's now in the lead.
Opponents in Metal Armor are the last ones where JHP has an advantage. Deathclaws also have resistances roughly equivalent to Combat Armor, so AP ammo is actually better against them as well.
Then you bring Power Armor into the mix:
The most damage a burst of JHP can do is 8-40.
The most damage a burst of AP can do is 30-61.
Against somebody in Advanced Power Armor, it's even worse:
The most damage a burst of JHP can do is 1-14.
The most damage a burst of AP can do is 26-51.
The AP Ammo is still truckin', while JHP becomes progressively more ineffective.
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AP Ammo is vastly underestimated by a lot of players, because they never really used it the way it was meant to be used. A lot of that is because the fact that it bypasses DT isn't mentioned anywhere, so most people look at it and think they're getting half-power ammo.
If you're devoted to the Assault Rifle or Minigun, then it's good to carry both JHP and AP and switch off based on who you're fighting. Carry mostly JHP, but keep a reserve of AP for if you run into stronger opponents.
As an aside, the 5mm-using Minigun/Avenger Minigun are the strongest guns in F01 and F02 respectively and nearly nobody knows it