This mod alters the AP and HE damage profiles a bit along with adjusting some of the armor values slightly. The goal is both to remain more true to what the UFOPedia entries say regarding alien weaknesses, as well as to ensure that both AP and HE are useful in some situations and not useful in some. This is not a rock, paper, scissors mod, so your favorite weapons will still be generally effective at exterminating the alien menace--even if your favorite damage type is HE.
Definition of Armor Piercing:Armor piercing rounds are heavy metal slugs with a pointed tip and a long powder cartridge, typically fired from a long-barreled rifle. They have high muzzle velocity and fly fairly straight and non-erratic. They excel at punching through rigid materials like tough armor, glass, rocks, bones, skulls, and concrete. They aren't so good at shredding soft tissues and may lack the stopping power of lower muzzle velocity blunt-tip rounds.
X-Com has scoured the globe for high-powered military AP rifles of varying sizes to ensure that their troops stand a chance of penetrating alien armor. Old WW2 rounds with modernized long powder cartridges are a favorite, and difficult to find in production because modern rifling allows much smaller ammunition to punch through even the sturdiest Earthbound armor.
Changes made by the mod:* Cyberdiscs are a bit more resistant to HE, as is stated in the UFOPedia. You can still take them out pretty easily with a rocket launcher, but the autocannon probably will struggle with them. On the flip side, they are slightly weak to AP damage. Their armor is too strong for rifles to be very effective against them, but heavy cannons using AP ammo or cannon tanks can be reasonably effective. With vanilla settings, powerful explosives are still the easiest way to take these out, but with more powerful AP weapons from mods, you might find AP to be particularly effective against them.
* Chryssalids and Ethereals, which are heavily armored units, are now quite weak to AP. Rifles will do a tiny bit of damage to them but heavy cannons easily take them out.
* Snakemen have strong armor, but I'm assuming it is fleshy like the Mutons. I gave Snakemen a slight weakness to AP, and it helps rifles kill them but you're still better off with laser rifles. I also adjusted snakemen armor values a bit to smooth them out. They were a bit erratic between ranks.
* I gave Reapers a strong resistance to HE. Due to their low under armor and large size, HE is still highly effective against them. In fact they still take more damage from explosives than smaller units do.
* Sectopods have half the under armor but take only half the explosive damage. The result is that mid-size explosives are still just as ineffective against them, but now blaster bombs deal less damage to them. It'll make them a lot more scary when you find it usually takes 2-3 blaster bombs to finish one off. (Or you can use lasers)