So PS4, multi player as guest. I killed 2 Apoc tanks with my .44 cal. Very clear it was my kill as the other player had not fired in over a minute on both tanks, No achievement was unlocked . Reporting as a bug. I will try solo play next. Oh it was on skirmish level
Update, I went solo, PS4 , killed a level 4 rival apoc tank , achievement unlocked.