In Reach, I never put two-and-two together in the final segment with the Mac Cannon. Keyes actually says "Those Mac rounds won't crack their shields, steady Spartan." It wasn't until I re-read some of the books the other day that I finally realized that this was because the Covenant lower their shields to fire their plasma.

You have to wait for Keyes' order to fire because it's the last possible second right before their fire their own plasma. Noble 6 literally shoots just in time.

Random and everyone else probably already knew this... haha