We get different numbers because of integer rounding. In my case it's rounding down.
I tested about 6 weapons of different categories and AP costs with dex of 14 and 16 and all numbers were correct. It does not mean my table is 100% correct, so if anyone finds an error - let me know.
But... what's up with the two different 5mm jackrabbits and 7.62mm jaguars? Does their base AP cost vary with quality nowadays?
It's a mystery to me as well. I've seen SMG with same caliber but different base ap costs.
I have 7.62 jaguar with 12 AP cost and 600 durability so it' probably not quality issue.
And another 7.62 jaguar with 12 AP with no attachments, so it's not an attachment issue.