User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-4721065-20130112104250/@comment-4721065-20130124041300

Firstly, it has been stated before in halo lore that sentinels have one centralised power source and have to divert it between speed, weaponry and shielding depending on the situation. Effectively, there is no way they would be able to fire their main ray at full power while remaining shielded, in which case there is no real point to them (and that is before we mention that they aren't going to be able to keep up the shielding forever.)

Regarding the gameplay mechanics, they're usually reflected very closely in "main" 40K canon (i.e. the codex & rulebook lore) and are also a far more stable indicator than the dozens of novels written by different people who take various liberties with central concepts.