[Mojo]

Yep yer totally right.
This trick can be done in SP and SP COOP.
In SP, when you get the pistol from Keyes, immediately run clockwise to the part where you first get the pistol, if done correwctly, the game shoulnt save when you reach the hallway when there's still the message 'Get off the Pillar of Autumn' on the screen. now, the grunts won't spawn. run though the doorway to get the assault rifle, and get any other gun. When you have two, return to the bridge, and when you see 'Checkpoint' walk back again. Now the grunts will spawn, and, if you cycle through the guns, you'll see you have 3 guns.

In SP COOP, get to keyes, and when you get to play again, have 1 of the players run clockwise, and the other counterclockwise to the hallway. When the person who walked counterclockwise comes to the T-section, DO NOT ENTER THE HALLWAYS!!! Instead, wait in the right corner. Meanwhile, the other person who walked clockwise runs to the right hallway, where you should get the pistol. If done correctly, you shouldn't get 1. Now have that player walk to the cafetaria, and get 2 guns, (preferrably a gun you can swap without losing a precious gun, like the AR). Now when you got 2 guns, walk back to the hallway, to player 1, BUT DO NOT EXIT THAT HALLWAY!. If done correctly it looks kinda like this

||**X****B*|WWWWWW|**B****|
||__*******|WWWWWW|*******| X= Player
|__|*******|__________|*******| B= To Bridge
|****************************| C= To Cafetaria
|****************************| *= open room, couln't
|****************************| get that many
|****************************| spaces
|__________********__________| W= wall
|WWWWWW|*******|WWWWWW
|WWWWWW|*******|WWWWWW
|WWWWWW|*******|WWWWWW
|WWWWWW|**C**X*|WWWWWW


Now, when the player near the cafetaria sees the other player, he has to kill him, so that he spawns next to the other. Now, get him two weapons. When yer ready, walk back to the bridge and wait for the chekpoint, return to cafetaria, et voila...