Say I start a character as a Chaos Space Marine. According to the character creation rules, I can choose equipment with a total modifier no worse than -10 (so anything -20 and lower is off-limits). As I read the book, this means any weapons of Very Rare (-20) or higher upwards since you usually ever want a single weapon, which adds +10 to the modifier. So, I choose to start with a Legion Plasma Gun (Very Rare), or, hell, even a Plasma Cannon (also Very Rare).
On my armor customization section, say I end up getting Power Supply Tampering and I choose the option to have an infinite power supply for my armor. With these options, I can choose to have my Legion Plasma Gun or Plasma Cannon connected to my power supply, which is infinite in power and never needs replacing, which means the weapon never needs to reloading and has no clip size?