Tiny hidden chamber in Doom 64 map 29, Outpost Omega:

Near the entry point on this level is a small hidden
chamber, just behind a computer screen/panel.  Inside is a
chaingun.  (Note that this chamber does not affect a
player's Secrets score.)

When you first appear in the level, you'll be facing down a
hallway, with a small rectangular room to your left, and
another to your right.  The chaingun is behind the fourth
computer-screen panel in the room on your right.

To open that panel, go to the opposite room, and go to the
third panel (with the dead screens) on the long wall, and
Activate it like a switch.  Quickly turn and run straight
out the nearest door, straight through the next door ahead,
and then into the cubby where the chaingun is, before its
covering panel closes again.