Stallion .44 Challenges...what am I doing wrong?

I understood that I need to get 5 kills with pistols and complete the match.

Several games I used a "Pistol Only" class, running the Kendall 44 Cartel w/auto sear. I went like 14-10 or whatever and completed the match but I didn't get any credit towards unlocking the Stallion. Not sure if this is important to mention but after getting the 5 kills, I finished the match with my other classes. Does this matter? Am I not understanding this challenge correctly or is this some sort of technical error?