It's really easy to see damage, example 10mm AP and JHP difference:
AP
AC modifier 0
DR modifier -25
Damage adj 3/4
JHP
AC modifier 0
DR modifier 25
Damage adj 2/1
So, you shooting with AP bullets, target has leather jacket (20% resistance from normal).
For example your pistol dealing 20 damage, so 20*3/4 = 15. 20-25=0 (i don't think resistance can be negative) So you dealing 15 damage.
Now you shooting with JHP bullets.
Again 20 damage. 20*2=40, 20%+25%=55%. 40-55%=18. You dealing on 3 damage more.
But if target has more %, JHP become useless. Btw, scorpions/geckos etc. has armor! They're not like bluesuiters. Look at wiki and see they have damage treshold and resistances. So JHP really good to kill bluesuiters.
So JHP deals 150% damage to 0% resist (x2 damage, 25% resist so damage*2*3/4), when AP vs 0 resists deals only 75% damage, so JHP will do twice damage vs bluesuiter than AP.
Alright, i think i might get my answer from that topic, but i still have no clue whats DR, AC e.t.c.
DamageResistance (easy to understood) and ArmorClass (% to dodge)