Well after watching that video above, I set my convergences to...
MGs & Cannons @ 100m
After practising online and offline, I found that I'm hitting things way much more now. Even when targets are 200m, 300m and even 400m away. Plus my lead shooting is way more improved too.
I guess the default setting of 300m was making me give too much lead and I was over-shooting the target.
I tested this fully in QMB with unlimited ammo and with regards to lead shooting, I found out that I was giving too much horizontal lead, and really what I needed to do was give more vertical lead instead. That surprised me. I guess the rounds got there fast enough, but gravity was having a bigger effect than I anticipated.
Horses for courses...