There are multiple issues at play here.
1) Hit registration - if you shoot all the rounds and not all of them even hit the target, you can't kill it.
2) Tick Rate - I cannot prove but I believe the hit registration issue is partly due to server tick rate. This really negatively affects the fastest firing guns in the game (ALL OF THEM!). This is why the biggest mags win.
3) Bloom, recoil and aim assist - I have yet to kill someone with the VZ61 in 1 mag. I have it at level 20, not hard but, as I said, I have yet to single mag kill someone with it. Now, when I die to it, from full HP to dead and it is a controller user, I know the game gave them the kill. This is because... Tick Rate benefits consoles, 25% reduced recoil and reduced bloom and aim assist, give the controller a distinct advantage over M&K and this is the result.
If all conditions were perfect, tick rate, hit registration and level playing field, you wouldn't need en entire mag to get a kill, maybe even get 2 kills in one 15 round mag.
I have yet to kill someone with the VZ61 in 1 mag. I have it at level 20, not hard but, as I said, I have yet to single mag kill someone with it.
Here on PC I have taken down foes with the 10 mag, but just a few misses and the other guy will get a free kill on you. It helps if I feather the trigger a bit.
