'Flanking fire', using unit arcs, is dead after poor testing. But that doesn't mean that I don't want it to matter how bunched up a squad is in front of a Machine Gun. So I've come up with a 'Sustained Fire' special rule that handles this instead.
So here it is:
Sustained Fire Rule
If a weapon has the Sustained Fire rule, it's capable of laying down a hail of bullets that will punish a a target which doesn't spread out.
When firing the weapon, lay down a standard teardrop template, with it's large end centered over the target model. The small end must then be pointed directly away from the firing model. For every model underneath the template, add 1 to the weapon's Rate of Fire (RoF).