Getting three weapons
After Keyes hands you a pistol in the cutscene, Bungie's programming can go a little screwy if you rush straight off the bridge without waiting for an exit checkpoint to appear. I've sometimes found myself facing Grunts with no pistol ammo. Then again, it's never been clear where the ammo comes from when you do get it. Sometimes the Grunts aren't even there, and my pistol seems lost without trace.
As part of this screwiness, it's possible to end up with three weapons to cycle through; and moreover, you'll have three weapon slots to use throughout the level. Yes, three and not two. I don't say this is any huge benefit - in fact it could even be a handicap because unlike normal, if you cycle away from a weapon then cycle again, you don't get back to that first weapon. You'd still have a cycle to go. But it's a novelty at least, so here's how to do it (and thanks to RedButton who emailed me some help to start with, as I wasn't sure how this odd situation had happened to me).
The recipe
When the cutscene with Keyes ends (you can just skip the main part), speed off to the mess hall as soon as your motion control returns. As long as you get clear of the bridge quickly enough, which is really no problem, you don't get the usual exit checkpoint, nor a pistol, nor any Grunts on the way. If you do get a pistol, that's no good.
After getting an assault rifle at the mess hall entrance, kill at least one covie and pick up a covie weapon. You now have two usable weapons, like normal.
At any time prior to the next loading point (which is way down the corridor beyond the wall-mounted medkit), return to the bridge. When you belatedly get the exit checkpoint there (missed earlier), head back to the mess hall. Before you reach it, the long-lost Grunts finally materialize with cries of "Yipe!" and suchlike, and you've also acquired a pistol in a newly added weapon slot. It doesn't materialize in your hand as you're already holding a weapon, but you can cycle to it. You've now got three weapon slots to cycle through, for the rest of the level.
Tip: When the Keyes cutscene is almost over, I recommend straining to turn, so that as soon as your motion control returns, you'll start turning for the exit to make a prompt departure as required.
Strategy tips
When I said to kill at least one covie, the easiest target among the first group of covies is the Grunt of course. If you want to get your three weapon slots as early as possible, I'd recommend killing him then waiting for the door to open. While the Elite or Elites are distracted by the new Marines, you can potentially run up to get the Grunt's plasma pistol, and either get clear again or attack from close up.
Bear in mind that when the first group is wiped out, that triggers the arrival of more covies at the far end, flocking in through two exit doors. They can be tough to deal with if you let them get well established, so it might be worth killing them before returning to the bridge. Blitzing them before they're fully into the room can work very well - such as seen in my kill 'em all speed run BCM22. You might also like to bear in mind that you don't need to kill any of the first group to get the new covies triggered. Advancing far enough towards the exit doors will trigger them - which is a great way of taking them by surprise and gaining the initiative.
Where's your ammo Keyes?
And hey, why does Keyes not keep his pistol loaded? Fat lot of good it's going to do him when the Elites come knocking. Later in the game, he further enhances his reputation as a bit of a silly old duffer by doing his best to rush headlong into the enemy and get himself killed. Don't you hate those 'Protect the cavalier nitwit' tasks that games sometimes throw at you?