>>370096071
I eventually broke and checked the numbers for the guns, and it looks like AP has 0.9 penetration value. Still doesn't make sense how I need to shoot an armored zombie like 10 times though, unless I was just really unlucky with rng. Also, it looks like the MP5K with HP does 80 damage while an AK only does 45 max. 
https://docs.google.com/spreadsheets/d/10Rp1w3GqUMl_2oSaMttJt7DsQSPhwZLVc7I1YQCp3Rg/edit#gid=0